Make them wear every piece of pvp/stam gear that they own. Do an attempt where noone does dps, and the only purpose is to hit the enrage timer.
If you crater, you get sub'd out. If you get hit by doomfire more than once in 2 attempts, you sit out. Archimonde is one fight where being stupid doesn't just affect one person.
Seriously, if people are cratering, sit em out. Its kind of like the ghost minigame on the Gorefiend fight, some people just flat out can not do it.
If doomfire is your issue, make sure everyone has their 2 min pvp trinket, and make sure they know not to burn it every time its off cooldown. They need to learn its not there to let them keep dpsing, its there as an OH SH*T button. If you didn't outrange the fear/doomfire and got caught headed for one, use the trinket. Otherwise, let the fear run its course.
Accountability. 24 people shouldn't suffer because 1 person keeps screwing up. your going to step on some toes, but start considering the fact that you might have people that need to be benched.
No excuse for cratering.
No excuse for getting hit by more than 1 doomfire.
We never had issues with decursing, don't think thats too big of a concern.
As for Bloodboil, I assume you mean your dpsers are threat capp'd? Or are people trying to squeeze in 1 last cast before moving and screwing up the rotations?