This Seeking Tech guide will explain how you can easily beat Retro Rampage 3 in Astro Bot.
For the first step, you should run past the two large enemies and then jump to one of the platforms in the back. Make sure you jump diagonally to avoid the spitballs from the slimes.
Once you land, defeat the two slime enemies quickly. You then need to jump diagonally to the other two slimes to avoid the spitballs before defeating them.
When you turn around, you may see one or two of the enemies remaining. You can attack them, or simply let them fall off the stage to end the level.