Assault Heroes

Somewhat of a harder achievement. Play this on the first water level, as the boss doesn't spawn lots of little enemies to ruin your percentage if you fight him in your vehicle. Just ditch your boat at the start and ignore it for the entire level until the boss.

I got this achievement just by playing (and failing at) the last level. Play through the entire last zone (area 14-end) and right after the final boss fight, you will put in an area where you must race to the end against the clock. Play through that like you normally would until the spot where it lets you reenter your vehicle. When you get there, do not enter your vehicle and play on foot until you reach the second health pack (looks like a wrench). Let the enemies kill you here. Repeat the timed area strategy 5-6 times before completing the game and you will get your achievement.

*Edit:you only have to do it 3-4 times. Thanks to bluphraust for the tip

Just a quick tip. I was attempting to get this achievement on Level 1. However, after various attempts I couldn't get it to unlock. Many others have also reported this as well, so getting this achievement on level 1 may be glitched for some.

Luckily, my first attempt on level 2 (beginning at area 5) worked fine :-)

After trying (and failing) in several different zones, I found this easiest starting in Area 8 on easy difficulty. It's probably easiest in co-op but I was able to do it solo.

The achievement description is not quite as informative as it could be. I originally thought that 60% of your kills had to be on foot. That's not the case. You actually have to kill 60% of the total enemies in the zone and they have to be killed while you're on foot.

At the start of the level make sure to press to get out of your vehicle. The key to the achievement is killing nearly ALL of the smaller enemies that can be killed using only a shot or two and then killing as many of the larger enemies as possible. I also made sure to destroy the large mines that show up but I'm not sure they count as enemies. Take the entire level slow and inch up bit by bit to cause the fewest enemies to spawn at one time as possible. Use grenades against the large enemies and save your nukes for the two bridges that stop your progression during the level.

The hardest enemies to kill will be at the second bridge. Once you destroy the bridge some small boats will come through. Try to stick to one side and destroy as many of them as possible. You won't get them all as they move off the screen too quickly. Larger boats will spawn next that you'll only be able to kill by tossing grenades in their path. I would advise you focus on avoiding their fire instead of trying to destroy them. If you have a nuke left go ahead and use it when there are several in the vicinity.

You'll run into a couple more waves of boats before you get to the boss. Go ahead and get back into your boat for him. He's not hard at all and I would definitely recommend using the Flamethrower for the entire battle. It destroys the homing missiles very quickly and makes short work of the boss' turrets.