If any of your dps die AT ALL before like the last 10%, you may as well reset it because you won't beat the enrage. The weakness is a HUGE hit to dps, and while it's alright (if not optimal) on tanks/healers, unless everyone else is absolutely 100% on top of their game you won't meet enrage with a dead dps.

Regarding aggro, have your cotank vokeshirk on cooldown (adds permitting). My drk ot vokeshirks me after the flame dies and I think at some point around books? It gets a bit hectic so I pay less attention. If your dps aren't using diversion/lucid dreaming/elusive jump/shadewalker/whatever, make them do it. Aggro can get a bit scary around about white flame for me (we have ot pull in grit, DA PS, then I voke off them) but the dps will be peeling off to hit the add and the vokeshirk should come right after.

You can sit in grit for the adds. The boss doesn't really do all that much that isn't telegraphed, so it's a lot easier for the mt to be in dps stance (and get a higher overall parse). It's not like you're hitting hali at that point anyway, and the adds won't die any quicker if you're out of grit.

If you're still having trouble and deaths aren't the issue, someone in your group will probably need to start parsing to identify your problem. Tank dps can shore up low raid dps but it won't win the raid if one dps is significantly underperforming.