Red Dead Redemption

The 10 kill streak must be done in the ADVANCED MISSIONS. I did this on The Herd. Get a game with just you and a buddy by voting to kick any randoms. Do this so you can restart if you mess up and you won't be competing for kills.

Pull out your sniper rifle and run down the first stretch of path that is downhill. Scope in and move around so the cliff side is protecting you and start poppin' heads. If you get lucky they will cluster up at a box of explosives near the tent on the right. Use the Marksman II class and you get 20 sniper rounds. If you lose your chain or run out of ammo you both can jump to your deaths to restart.

You must complete each mission once to unlock the advanced missions. In this mode you are required to use expert targeting mode and enemies deal/receive more damage. So unless it's a head shot, the buffalo rifle takes more than one shot to kill. Also it helps to have your Coop partner stand behind you in case you get downed so he can revive you in time to save your chain.

Mounted weapons like the cannon work well as long as your immediate area is clear of hostiles. The Escape and Ammunition provide ample kills on the cannon. The cannons in The Escape are on the hill to the left of Tumbleweed as you ambush which is about 10 minutes into the mission. The cannons in Ammunition are on the Southern Hill and require killing over 20 soldiers to mount the guns. The setup time is much longer than on The Herd so try not to make mistakes if you choose the cannon route.

This is by far the easiest method.

1: Do the ESCAPE ADVANCED MISSION.

2: Progress through the mission until you get to tumbleweed.

3:you will notice on top of the hill over-looking tumbleweed there are some conviently placed cannons there. take out the people at the hill with your buddies and then use one of the cannons.

4: Now just kill the enimies below. Aim at the town area first theres about 8 or 9 people there, then you will see about 5 or 6 people around the church, shoot them.

AND OOH WHATS THAT? ... ACHIEVEMENT UNLOCKED!!!

(REMEMBER THESE KILLS HAVE TO BE IN A CHAIN)
this can also help you towards the mowing them down achievement getting 500 kills with a mounted weapon.

Achievement guide for "Dodge This" 10 kill streak on an Advanced co-op level.

This is the easiest way, do "the herd" in a private co-op with a friend and "marksman II" class...

Thank to leadexample for the video

This solution contains some small optimizations to JsNuke's solution, in case you are as lousy at sniping as I am.

Get a 10 kill streak in ADVANCED MISSIONS. Start a private free roam, invite a buddy, start advanced missions. Skip missions until you get to THE HERD, start it and select the MARKSMAN II class.

Once the mission starts, crouch and move to the edge of the cliff;
Zoom in on the tent with the TNT and a guy taking a leak right next to it, it's on the right side of the camp;
Don't shoot the TNT, but shoot the guy next to it instead;
For some reason al his buddies coming running towards him (and the TNT), shoot the TNT when it's crowded and start your kill streak instantly with 7 or 8 kills;
Get the last few kills with the sniper;
If it doesn't workout, you and your buddies jump off the cliff and start over.

Thanks to BillJones0302 for showing me this method.

This achievement can only be won on Advanced Co-op levels which gives you harder levels and expert targeting.

The easiest level Ive found to do this on is the Kidnapped Girl. Choose to go the build of Marksman II for this level.

Immediately pick up additional bullets in case they are needed, then stand behind the rock to the left, its the one thats behind another rock. With all your shots you must aim at the head, if you hit the body it could take 2 shots to kill, which wastes you precious time and can ruin your streak.

Take out the two men on gattling guns first, left then right. When you kill someone on a gattling gun then another man will usually run to take their place, so by killing these first you can set up two easy kills later in your streak.

After killing the two gunners look at the top right of the building where you'll see a man looking aimlessly to the left. Kill him. Then move across and aim at the black guy to his right. You may have to duck down between shots. Continue along the top of the fort, killing a man, moving aim across then ducking down.

When you get to 5 kills you can now look back down and there should be two men on the gattling guns again. Kill them both.

Now look to the far right of the fort. You'll see a man in the distance with no cover, then a man slightly to his left on a lower level who is under little cover. Kill both of these and then look slightly to the left where there will be another man on top of a tower under no cover. That should be your 10!

This is not a difficult achievement to get, and by replaying the start of this level you will learn the places the enemies are and will find it easier.

I tried a lot of different methods to get this but i finally got it after some advice from a friend. start up ammunition in private mode with a friend run straight forward select dynamite and turn to the right throw a couple of sticks through the gate at the end of the town then swing straight round a do the same to the other end if you nail it right you will get 6-7 kills from the first throws and 3-4 on the second ones.It took a few tries as you die pretty quickly if enemies get through the gates but once you get the timing right it should take about 5-10 mins to nail it.credit goes to brayarg trueachievements.com for passing this on to me

You probably know this by the time it's even possible to get this achievement, but kill chains are obtained by killing one person after another with just a few seconds between each kill. In the top right corner of your screen you'll see a green number denoting how long your kill chain is (assuming it's at least at two) and a green circle behind it that will slowly disappear. When it goes away, your chain starts over at one. You have to get this achievement on the Advanced missions, where you no longer have snap-to aiming.

I tried the other methods. Seemed like I always fell short when going for snipes, someone would shoot me and end my combo, or (this happened a lot) I'd just take too long to find my next target.

If you have trouble, try the mission "Walton's Gold". I feel like I should put a spoiler alert here, but on the other hand you should have played this through on the non-Advanced version already. Plus, there's really not a plot to spoil, is there? Nevertheless: "Spoilers" follow.

You'll need to start with dynamite, so select either Miner II or, preferably, No Holds Barred as your loadout. Clear out the town normally; I prefer a sniper rifle until I run out of ammo. Once that's done, equip your dynamite. Have your teammate enter the mine behind you. When you get to the first stack of TNT - it will be on the right side of the path in a bright red container - take cover behind the crate that is just before it on the left. Have your partner stay behind you and shoot the TNT. Be in cover when this happens.

When it blows, people will start milling around and head your way. You can take the brief time before they show up to get your aim just right, aiming for the right side of the "Y" intersection directly in front of you. You don't want to be holding the dynamite and trying to perfect your aim after they get there. It blows up in your hand if you wait for too long, and sometimes they'll shoot it themselves and kill you. When they start to show up, wait until a good-sized group assembles, then toss the dynamite into the group. You'll get four or five, then another group will arrive. Blow them up. Keep going until the achievement pops. In three tries we managed to get it twice, both times getting the multiplier to 14. If for some reason you did fall short, you could equip a rifle and quickly head around the bend and try for some quick headshots, but it shouldn't be necessary.

This is an expansion of Lockyz75's solution above, so credit to him and brayarg. I started out writing a comment on his solution but it was so long, and hopefully of use to people, that I thought it might not be seen there. So I have voted his solution positive and put my extra details here.

You should be aware that it might not be as easy as the original solution above makes it sound. Two of us started this at 4:15pm and by just after 4:25pm one of us had it. At 7:05pm (!) we finally quit with the other - me - unable to get it. Almost to be expected, my last attempt achieved a chain of 9 (for the second time). :(

We did learn a bit along the way though...

If there's two of you doing this, one of you will start closer to the central road than the other. If you're the guy at the back (me) then the extra 3/4 yards you have to run means you're f*cked on timing. It's simply not worth trying from here and you need to do something to get the better start position.

We tried restarting private sessions and skipping through all the other co-op missions, switching who was the posse leader, but none of it worked. Although it could have been a fluke what did it in the end was switching characters. He had been Luisa Fortuna and I was Marston; when we swapped I was close to the road at last.

Be aware that the enemy spawn is not the same every time. It is the same every time you retry immediately, but if you've been away to the main menu and/or rotated back through the other co-op missions then it might not be the same as the last time you did it. For example, when the other player was close to the road, throwing one stick to the left of the west gate and one to the right usually got him 6 kills, for me it got me 2 a few times and often none. When I got my chance starting close to the road it became obvious that a stick needed to go to the right first as the soldiers were definitely not coming in the same way. This still only got me four kills though, to get six I needed to get throw two sticks to the right of the west gate and one to the left - pretty hard given the limited time available and needing to address the east gate asap.

Lastly, don't be tempted to use Miner II for your loadout; always go with No Holds Barred (if you have it). You already have to hold LB while pushing the right analog stick up-left to select dynamite on Miner II. Doing this on the other loadout would give you the Tomahawk, but all you have to do extra to equip the dynamite is keep holding the analog stick in place for a moment longer while quickly tapping right (or left) on the D-pad twice with the thumb of your left hand. As you can tell from what I have already written above, I was sh*t at this achievement (and still dont' have it) but even I could select dynamite in no more than a second when it wasn't the default option in the slot.