This is the second boss. He is a robot centipede that starts with armor on. It is useful to use your specials against the armor and then shoot the hell out of him while it is down. As you drop his health he will go through three phases
1st phase: is normal, he launches little one shot enemies at you
2nd phase: he starts launching harder to kill enemies and leaves trails of land mines.
3rd phase: he will stop summoning troops and start launching rockets. This is the easiest part IMO because he is so small and aggressive that you just need to circle him shooting.
Video clip of my speed run (Note I am very strong, level 65 damage and level 50 health) The whole video is 1 minute long:
He will blow up and the achievement pops. Get some practice on all bosses, to make the 20 levels without dying achievement a ton easier.