In previous rare quests are you attempting the last stage and dying or are you skipping it? If you are dying, when? Are you having trouble getting through the mobs, or does it feel more like you just don’t have the hitting power to put the bosses down?
I may change my response after hearing your answers, but my first thought was Sabina, Boldtusk, Balthazar, Caedmon, and Evelyn.
Since my primary concern is the bosses I do like your idea of a triple stack purple and I think that’s probably viable. I guess my suggested lineup favored putting your overall most durable heroes onto the field. It won’t pack as much punch on purple tiles, but Evelyn’s elemental debuff and the same fast mana speed as Caedmon should mean you can often fire her, then Caedmon. In my suggested lineup I’m relying on that being pretty effective for you.
I also was tempted by Spirit Link, but I don’t like Kailani’s color being weak against so many of the mobs (and more importantly, to me, the bosses). If you have space I’d try to replace her by bringing Turtle Banners.
Which brings me to items. For me, Antidotes are a must to offset the bosses special skill. Even with two healers I think you’d be wise to pack healing potions. The last two spots are tricky, as I see a legit argument for a weapon, mana, and turtle banners. I think I’d make this decision based partly on your play style and risk tolerance. Are you going to kick yourself if you use bombs and lose? Or if you forego them and lose but maybe could have won if you brought them? Or are you a gambler such that you can skip antidotes in favor of weaponry but live with a special missing at a critical time?
Ultimately, the team I suggested has a power of 2625 (before troops), which I don’t think is that bad for a stage with a recommended team power of 3060. You can probably do it if you follow the usual advice of charging up your specials before each new wave, unloading items (if you’re going that route) focusing on a single enemy, then ghosting. Good luck!