You need to reach level 6-1 to fight this boss.
Easiest way to kill this boss is to have a ranged weapon like crossbow or sniper, go to one of the corners and attack the center. If an explosive drops where you are, go to a different corner and repeat. If it drops somewhere else, just stay and keep attacking. Depending on char/weapon combo, you should be able to kill it within 1-3 explosive drops max.
The boss fight is shown at 19:08 in the video.
Full video achievement walkthrough: