More Like Wrecked-us

I couldn't find any actual strategy to do this, so had to figure it out myself. Here it is... you need to zerg the boss. he will get to 1 health and he won't die until he casts Tectonic Upheaval. He'll split into 2 smaller versions of himself. Repeat the kill. He'll then split into 8 versions. Kill them all at the same time and you'll get the achievement (after he reassembles.) DON'T stand on the swirly things. They'll one-shot you.

Oh god, why.

I found that the posts here are helpful but not quite precise enough, so here is how to get this these days:

1.) Kill Tectus, you can 1-shot him, no problem, he will not die but split into two smaller copies.
2.) Kill the two smaller copies, again 1-shotting them, no problem. They will split into 8 smaller copies.
3.) Group up the 8 copies and kill all 8 small copies as fast as possible. That's it.