Turn-based would take even longer from stealth, though with some builds combat is indeed very easy, given you can do infinite free actions, including a lot of potions, many wizard spells, etc. Like tactician / skaen and tactician / blood mage are both super easy. The enemies also don't move as much, so possibly they wouldn't find you in stealth. Still, I'd like to see this guy beat Dorudugan from stealth...but yeah, possibly since you can see which attacks he is going to use you could position the summons so helfire barrage always misses, or maybe just break the AI in some way so he just stands there. Turn-based mode seems more of an afterthought, not very fleshed out compared to games built for it, and unbelievably slow. The one turn-based ultimate submission didn't do Dorudugan in turn-based because it apparently takes ages, though I watched some of the other vids and it looked very easy.
EDIT: Tried Dorudugan turn based with spiritualist. His stealth detection radius is smaller in turn based so you can stand quite close, and you have longer to reach if you're starting to be detected, and he doesn't really chase you. So...seems much easier to stay in stealth. But he still does his helfire barrage and I couldn't get him to direct it at anything in particular, he just fires the fireballs in all directions around himself which does maximum healing damage. Seems even harder than RTWP for this one because I can't figure out how to avoid his healing, but I can believe the other megabosses might be doable given the difference in how enemies seem to react to you being in stealth...
Mmm....Belranga is the easiest megaboss to kill with a troubadour IMO. That one you can just stay stealthed and send in summons and the spiders will eventually kill her (can do same with monk with enduring dance and dichotomous souls), but yeah Dorudugan is probably next easiest, assuming Abydon is off. Hunting bow modal and essence interrupter indeed shreds him. I did kill Dorudugan with all the challenges on using a devoted / troubadour with legendary war bow as my weapon and hot razor skewers plus energized which got me to 18 pen. For some reason the non-unique weapons don't degrade.
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I tried the missile gloves to proc Sea Stories from Whale of a Wand. Doesn't work unfortunatley. I cast it 60 times, and it procs 6 per hit, so the probability of none of those 360 hits proccing sea stories is .95^360 = 9.56 x 10^(-9), so...yeah. Doesn't work sadly. Also does a lot of damage, I did 140ish a couple times with just 16 might, and she has 163 health at L20, so could possibly kill her. Ray of Fire scrolls work well and are relatively plentiful at least. But I wouldn't cast them with max arcana, I tried it and she died lol. With < 10 arcana should be fine, I never got her more than bloodied with 7. Can carry healing scrolls just in case.