A bit part of the problem with big battles is that the mobs constantly drop out of combat while you are fighting them. This not only clears dots and debuffs, it also drops you out of combat and makes builders not count.
The best suggestions I can make are:
1) Forget trying to build gambits while fighting, just spam spear/jav builder.
2) Run with someone who can blow up the mobs well - bonus if they can also heal NPCs. So yeah, mini, LM and RK are good (though I've heard RKs also have attunement issues for similar reasons to our gambit failures). I sometimes run with a great guard, but we still have problems from not being able to toss heals on the furniture and whatnot.
Also, if you can use them ballistas are pretty good for some of the side quests, much better than failing with the assailment skillset.
Gremblus, Seuer, Grelob, and various others of Arkenstone