The Ace achievement is pretty hard. With 4 players, it's doable though. You have to play on insane to get enough enemies on the screen. It's easier if you have one person do most of the shooting to collect points. This is my strategy:
Have player 1 collect all the offensive weapons like the homing missiles and the laser. Player 2 and 3 should be combined and collecting the EMP and shield powerups. When player 1 has a weapon, he should connect to 2 and 3 to use the weapon (makes it a lot bigger). Take out the mines too, they give you extra points. You will see the laser can clear out 2/3 of the screen now instead of maybe 10%. It's helpful to have a player 4 that stays out of the way and respawns everyone if players 1-3 all die at the same time, which happens if you get lasered by those purple guys.
Hope this helps. It's hard to describe the game without knowing what those weapons and enemies are called.