On the bosses:
"You are not obliged to kill that boss to win the dungeon (at least with the crusader. It might have to be tested with other classes)"
I've tested the Cursed Oasis with two non-Crusader classes, and can confirm that only the Curse Dragon needs to be slain to win the dungeon. What has worked for me best to consistently achieve this is to worship the Earthmother, whose Greenblood boon removes a Curse stack for a fairly low cost (assuming 5-6 strikes against the Curse Dragon, it's only about 38-55 Piety, which is easy to rack up given the abundance of Corrosive Eelroots in the curse realm). Between stoning the plants and using Plantation I usually collect around 250 Piety this way, enough for the 5-6 Greenbloods, 2-4 Clearances, and 8-10 Vine Forms. One watchout is that once stoned, the curse realm Eelroots start existing in the normal realm as well, so first collecting whatever is underneath (or having a way to break the stone) is recommended.
Is the no subdungeon thing true for any version? There are subdungeons on the iOS and web versions.