You say you had a nin/war and a war/nin... so you had two tanks. Unless I read that wrong you were saying you bought time for the war by tanking with nin... I meant if you could only have one tank for the fight, nin/war would be the absolute worst you could go with (next to anything else/war)
Yeah one error even in the best party can throw it off, I know, I faced my 2nd and 3rd losses ever last night, once because of a stupid mistake I'll take credit for, and the other because we only had a nin/war to tank holla. The 2 hours don't necessarily win the fight unless the bosses are dangerously low. You'll hear countless stories of how the boss gets to "almost dead" and its because after using their 2 hours, they couldn't dish out more consistent damage. DoT makes up heavily for the drain abilities these bosses have, especially since they take chunks of HP at a time. RNG/NIN, though you're right. Absolute best you can have for these fights. More RNG = safer fight, without a doubt.
you do not have to kill the bosses fast, except for at the end,
I said for the final stretch, more consistent damage faster is better than one large shot with no re-use (like Eagle Eye Shot or Astral Flow, which has a minute downtime in between each shot from it). Hundred Fists will rip through any regen a boss might have, or any hp drains they use, and keep it suppressed.
Funny story enough, you may think DoT is not an issue, but in a last ditch effort, a simple thing like Dia has made the battle for my party. We were literally all dead, and the only thing left was a Dia the WHM cast on it before he died, and it falls over dead as it comes to claim its last victim with 5 HP left and no MP. Heh... probably was pure luck we pulled that off but hey, without that Dia we wouldn't have done it.