The other guides here suggest doing this achievement on Chapter 5-1, but after a few unsuccessful attempts I tried chapter 5-2 and got it in one try. There are a lot less nasty surprises and most large fights are preceded by a powerup spawn.
The advice remains the same: inch forward until you can see the tips of enemies at the edge of the screen and use your rockets and grenades to shoot them until they're fully spawned. Take your time, if things get hectic, concentrate on staying safe and be careful not to get backed into a corner. Do not attempt this unless your walker is fully upgraded (or better yet, use the Mega Walker.)
Try to leave your spark bomb until the end of the level which has 2 giant tanks, 2 flying gunships and then a final giant tank. You'll find a grenade booster spawns just before this fight, so here's how to do it:
- Pick up the grenade booster and destory the first two tanks (the grenade booster will actually remove enemy bullets from the air)
- Once both gunships are on the screen, destroy them with your spark bomb (Y button)
- Use your rockets and grenade to take out the final tank. He shoots a stream of bullets at you, so don't move around too much, else you'll widen his firing arc and it'll be harder to dodge them. EDIT: I found on my hard playthrough that if you're able to hold out long enough, it's possible to hit the two gunships and the last tank with one spark bomb, but at increased risk of getting hit.
Good luck and if you get hit, don't get frustrated - continue to the end of the level and then restart, that way you'll learn the layout of the whole level.