Contra

Always keep the Spread Gun, try to stock pile lives so the end boss can kill you a few times. Watch where to stand for some of the bosses. You might have to adjust but the locations are decent. Learn a path that is good for you. The last level do not get any pickups so you can get the barrier about half way through the level it is a big help.
My video helps showcase a good path, the barrier thing, and beating the game in under 12 min also. Please enjoy and i hope this helps you
***UPDATE AS OF 6/21/11***
The Barrier in level 4 requires at least 1 death in the level before the area that it shows up. Also the video is done with out a turbo and I just tested the achievement as of 6/21/11. The achievement is not glitched. Maybe the way with two controllers is but my current guide is not glitched.

Another solution that you can use is log in a second offline gamertag as player 1 (we'll call him player 1) and then log the gamertag that you want the achievement to go to in as a second controller. Begin the game regularly starting with only player 1 playing use him to get as far as possible. You should be able to use him to get to at minimum the start to middle of the last level. As soon as you are down to your last life and are near death press start on your second controller and finish the game with the gamertag that you wanted the achievement to show up on. So in essence doing the game this way you basically start with six lives. This method works I have seen it done but I just can't quite get far enough into the last level and I am always just 1 life short of killing the final boss. Given another week of practice this achievement will be mine.

The other two solutions have solid advice, but some of us just aren't that good. If you're like me and want a solid 200/200 on this game but just can't get this achievement legit (and if you have plenty of free time), there's a way to make it a whole lot easier.

On the last room of the second tunnel, crouch directly in the middle of the room (your bullets should be hitting directly below the target) and just keep killing stuff. Don't exit, just keep crouching and pushing A as fast as possible. The time will run out and you will die, but for each full run (from the time the clock resets until it runs out), you will gain about 80,000-85,000 points - enough points to give you +1 life and a little over. Repeat this long enough, and you can get as many lives as you need to beat the game.

I did this for about two hours and got up to 15 lives... plenty to beat the game with and get the achievement. Like I said though, it will take a lot longer and your hand will be killing you towards the end, but it makes the achievement so much easier.

alle these tips are great. So all im going to say is, Get a Turbo Controller. with a Turbo controller even the start weapon makes a lot of damage in fact it make so much damage that only the blaster (the one that shoots rapidely when you hold the button in) is useful.
(if you have problem understanding what im writing then write a comment and i will change it)

Well, if you dont have a mod controller like that video, then heres a way easier way to beat it.

hey guyz, i thought id tell you how
got my *Bulletproof* achievement.
by going through the level

again, if you die at ANY part
that I don't tell you to die
at, then you must restart.

level 1

Start killing the people in front then
while running forward shoot up(never down)
to summon the spread power up.(S) marked.
I say never down because its a crapy
laser and sometimes when grabbed
the spread doesnt appear.

Once you have the 3 fireball shooting
power up, Press and hold down and hit (B)
to land in the water. while in the water
continue running and aiming up and forword
(to the right upper corner of the screen)
Tap (A) as much and as fast as you can,
laying waste to everything and everyone.

After the little island, continue in the water
till you reach dry land. now aim forward
because theres a guy thats going to shoot.
lay down if you dont kill him in time.

after this point just kill everyone with
the spread until you get to the end
(another power up will pop open and
land on the platform above you.

BOSS
Ignore the power up but jump on the cliff
above that its resting on.(you should be in front of it.)
now jump in place and shoot straight at the
guy on top.if done right he will shoot up and miss you
if not, then jump again, avoiding the power up and the bullet!
there is no need to kill both guns. Just lay down where
you were jumping and kill the fist turret. once killed,
fall down right under where you were laying.
kill the door and level 1 is complete.

Score est:37k-42k

tunnel 1
There are 3 different types in here.
1.Skipper(skips or runs)
2.Butt buddys(mobs of men)
3.Yellow "F" bomb(Yellow guy)
That best describs there attacks lol.

1.shoot the middle asap fast destroying it
before enemys spawn.
Skipper shows up on the right side.
Take a right at the end of the hall.

2.Shoot the door switch on the right
asap and no one will spawn.
Skipper runs across the screen from
right to left, but shooting at the
switch will kill him.
Take a left at the end of the hall.

3.Note** 3rd part in both tunnels have different
spawns, but you can take out this one before
they do so. just stop shooting if you spot
a yellow guy.
take another left to the final door.

4.Lay down in the middle. start shooting feet.
when the blast shield comes off the door switch,
stand up and fire!! if done right, you can kill it
with out moving, but if you see bullets, lay down,
and wait for them to pass your feet, stand up fire 2-3 times
then instantly lay down again.

BOSS

This boss is hard to explain, its basically 6 squares.
you want to go to the edge of the left sqaure.
heres a diagram. G= squares L= you
G G G
G G G
L
where the top of the L is, that is where your head should be.
Ignore the bottom of the letter.

If done right, You should be in a place where the 3 bullets cant
kill you. with the spread it should also be damaging the bullet shooter.
once the left side is gone, run over to the right side and do the same.
EXAMPLE

G G G
G G G
..........L
stand here so you dont die!. once this is complete finish off the
boxes and a orb guy that shoots fire comes out.
DO NOT kill him! in fact we will be weakening him.
Shoot up and count to 4. stop shooting and get hit
once. when you respawn, move to the dead center
and tap a as fast as you can. it will kill all fire
heading to you and if it doesnt blow up the attack
step outta the way which ever way he shoots it.

(5% chance to get hit if they dont blow up)
(.00000000001% chance everything turns to chaos)

Score est: 72k-80k
(based on how fast its done)

LEVEL 2

now that your weaponless, you can get the power up
here.

Jump up and lay down fast, shooting the baddies.
keep laying down and tapping (A) until both power-ups
get shot. get up once all the bullets are not above you.
If done the right way, it should land on the moving rock
above. if not, then its easy to jump down to get it.

It's the (B) power up. its like star power.
heres the tricky part. once in this state, you
must jump up to the VERY TOP.

Climb fast, remember,no one can hurt you.
Before reaching the top, there are 2 big rocks that fall down.
at the 2nd hole, lay down and shoot the power=up box.
its a spread and should land on the moving rock platform.
(1% chance it will fall causing you to miss it)

if you miss this power up, then you must restart.
jump to the top and jump again as fast as you can until 2 guns appear.
Your still immune to everything so take out the sentrys and the middle
multi-cannon. by the time you take those out, shoot the guys that run
to your left, then right, your not immune for much longer if already not.
Destroy it and move on.

Tunnel 2

Before I start to explain this, I must name
the people in here.

There are 3 different types in here.
1.Skipper(skips or runs)
2.Butt buddys(mobs of men)
3.Yellow "F" bomb(Yellow guy)
That best describs there attacks lol.

Part
1.you Must jump and shoot on the right side to
break the door switch. Skipper comes out
followed by the butt buddys. should be the only
guys you see before you break the switch.
Take a right at the end.

2.Dead middle. skipper is the only guy you should
see if you go right for the switch.
Take a left at the end of the hall.

3.This one spawns different people.
luck is good, hope its skipper.if not
its the yellow "f" bomb and instantly stop shooting
or your going to get a laser!!
The switch it in the middle on the floor,
just laydown and shoot.

4.Jump and shoot in the middle.
2 skippers will run at the same time hitting
the backs of the blocks until they get to the middle.
lay down if you see a bullet about to touch your feet and jump
and shoot. the yellow "F" bomb will show up or the butt buddys.
randomly.
Head right at the end of the hall for the last room.

5.lay down and start breaking the block.
Watch out for skipper, he sometimes throws a nade over the block.
next guy is another skipper followed by some butt buddys.
some times the game will throw the yellow "F" bomb in the mix.
(dont shoot him, or you will drop the "F" bomb.)
after the block is gone, stand up and shoot then lay down.
use the same as the last room in the last tunnel to break it.

BOSS

Note** if you die at any time I didn't tell you to die at,
you must restart the game.**

Again, my camera doesn't work but the boss is kinda like this.

G= Cloner C= Multicannon L= you

.....C

..G.G.G

.....G
...L-->L
the spread is good for this part.
Basically move from left to right avoiding
the easily avoidable multi cannon and destroy the
cloners (G) asap then the cannon.

now finish off the little people and perpare for the boss.
O= skull orb guys. L= you (())= bad guy movement.
.((O))((O))




...L<---L

start at the right, right under the skull and fire!
the spread should kill the attacks he throws, if not move to the side
an inch and look up again. move to the left and repeat until
you finish him.

Score Est:155k-165k

FINAL LEVEL

finally!!!
start off spraying your spread and run forward.
once you see the space ship> SHOOT it down.
watch out for nades!! there should be 2 parts that
spawn nades right here. dont jump over, wait. then go.

This part is the hardest part in the game.
when you get to the first drop off, wait and kill the
guy flanking you. then jump down** NOT FORWARD**
slowly inch forward and youll see a guy on a turret.
turn around, back into the corner and DUCK!!
then while laying down, wait
for the bullet to pass, then stand up turn and dont
move and lay down so your facing him.

if you just lay down, a guy will spawn and jump on you.
once that guy is gone, kill the guys above you and inch
your way forward, yet another turret guy! WTF.

this time, you must duck, then instantly jump
over the guy that WILL spawn behind you and the dodge
the bullet at the same time. when you hit the ground
instantly lay down and shoot shoot shoot.

past this point is pretty much cake.
run and shoot everyone in site and watch for
rockets.

once you get to the tanks kill em both.
now this is another tricky part.

I call him, the juggernaut!
after the 2nd tank, creep forward, and kill flankers.
once the sky turns night run as far back as you can.
turn around and SHOOT, mash (A) faster then you can mash it!

the juggernauts moves are simple. you get too close,
he drop kicks you. far away, he does 2 attacks.
a DEE da Dee jump and throwing saw blades.

jump over the saw blades as much as you can and NEVER
stop mashing (A).

once he blows up. jump down and watch for fire
ill give you a hint, the fire doesnt flow
unless you go. so dont wait for it, move, nudge, inch
your way.

::HERE IS THE EASIEST ROUTE::

JUMP down and lay down, move lay down. until the fire
blows then run. shoot the guyz outta the air and jump
to the middle platform. once you shoot the laser
power-up box, it will fly to the top.

jump up to the top AFTER the laser power-up
and inch your way to the top fire shooter.
this will be shooting down and after it shoots,
you want to get to the VERY edge and jump over
the bottom fire shooter.

when you jump to the next platform. inch your way again.
the 2nd juggernaut yay!

when you see him, intantly turn around and fall into the crack
left of the screen, you cannot be touched by him here.
shoot up and over to kill him and move on.

now you make your way to a spike pole kinda room.
kill yourself on the top spike pole and move to the right

you will see a (b) power up orb fly above you, shoot it.
once in star power mode, RUN!!! FASTER, damn screen is laggin.
okay keep running, then shoot every guy in your way
with out stopping though. once you reach the alien head.
jump under its jaws and shoot up. if your fast enough
you can kill it before it kills you.

inch your way now and shoot the faces that shoot balls
at you.or egg sax i cant tell. kill the ones on top until
you see them on the bottom, then run like hell and get to
the last boss. jump shoot, get lucky. anything to increase
your chance.

BOSS

the final boss has eggs on top and bottom.

F=face E=eggs H=heart L=you
_______________
|...............F.EEE.|
|.....................H..|
|.....................H..|
|L................EEE.|

when your forced into this small room. you want
to lay down in the farthest corner on the left and
shoot. it will kill all the hatchlings that fall and
also kill the eggs under the heart. when you see the
face let off one of those balls it will hover and home
right into you. avoid this by jumping and shooting it.
intantly lay down after its dead.once all the eggs under
it is dead, you jump and shoot the heart avoiding death
if possible

Score est: 1 million and above.

sorry, the site messed up my graphs
Update, I managed (tried to) fix the Graphs.

Also I might beable to make a vid if someone msg me about it.
(new cam)

I'm late to the party, but I found a video which I thought was very helpful and wanted to share. It includes commentary on some of the more difficult parts, especially toward the end. In the final level, he tells you how to get a barrier which makes the 2nd to last boss a total joke.

Make sure you follow his advice at the beginning regarding the two accounts. Your dummy profile should play through most of the game as 1st player. Only sign in your real profile toward the end. This essentially gives you a few extra lives without voiding the achievement.

If you're fast enough, full assault will also unlock for the 2nd (main) profile using this method.

Best shooter just don't let go of the trigger mainly repetition