Deep Sixer not awarding
Been through the fractal multiple times getting it to 19 without going to 20 multiple times on each run, and it still will not award the achievement. Either there is something extremely picky about how it works that no one has bothered to write down or the achievement is broken.
The 21st stack is the one that makes him explode iirc, so you have to get to 20, then stop. Not 19.