(From RCATT, Contributed by Keith McKnight / Mark Brereton)
Here's a great cheat for Slap Fight. Insert a credit and select 1 player. When you begin the level, just sit there - don't kill anything. Let your player be destroyed. When you begin again, you'll have all weapons and powerups!
If you don't fire at all, but dodge your way through the enemy shots as long as possible, you'll get a big bonus and start your next life just before the second big boss. You have to get as far as the ring shaped pool to get this.
(Comment from KLOV entry)
At the very beginning of the game, try staying alive as long as possible without shooting any enemies. You'll eventually get hit, but your next ship will have maximum fire power. The longer you can survive without being shot, the larger your bonus will be and the farther along you'll restart.
At certain points in the game, voluntarily changing to the basic weapon ('shot') will sometimes cause a system-controlled assistant spaceship to appear and give you a helping hand. The exact circumstances that cause this vehicle to appear are not clear. I believe the automatic player is known as 'Bob' or sometimes 'Bod'. 'Bob' attacks all enemy targets, but is susceptible to their fire. His kills count toward your score. It is possible to preserve Bob's lifespan by using your spaceship to block him from going into danger.
The small 'sprouts' that can be seen at some points in the game can be made to grow into a tree and bear fruit by shooting at them with the 'laser' weapon.