lower resolve doesn't seem like it would matter. dorudugan just has an enormous amount of health, that basically nothing matters except just damage or resource regen simply because the fight is so damn long.
and wow, i was going off the wiki which put dorudugan's health at ~6k (~7.4k on potd). no wonder my 200+ stacks of resonance didn't do much on my last attempt... way for the wiki to be off by an order of magnitude.
this is my general opinion on dorudugan. I've never heard Titan Souls, but it sounds about right. The hardest part (and the "trick") of dorudugan is just being able to position your characters appropriately to avoid the fireballs and the ground explosions, and to gear up appropriately to survive the vacuum/fire explosion attack at <bloodied. the fact that dorudugan has so much health just makes it tedious, when most players have already demonstrated that they know how to survive the fight after a few of each attack.
maybe dorudugan should have like half the health and slightly lower defenses, but use their abilities more often. players would still need to handle the main "tricks" of the fight, but they wouldn't have to do it over a huuuuugely long play session. with lower health and slightly lower defenses, maybe some buffs and debuffs would actually be useful without needing brilliant backup.
edit: as far as the other megabosses go, i dont' think they need much tweaking. my last attempt was slightly more optimized (using a monk and a beckoner) and i could clear hauane, sigilmaster, and belranga rather quickly (wotw with ajumaat's stalking cloak is stupid against hauane). however, even being more optimized for megaboss content, there's just no getting aroudn the fact that i have to set aside like half an hour to an hour just to tackle dorudugan, even though the basic trick of the fight is muscle memory at this point.