I don’t recall weaknesses off hand. But I would typically have about 3 more cleared areas (. . . ish; I had done Orcs, Abandoned Farm, and pushed into several other places before coming back around to Dwarven Temple) by the time I did him. that’s probably worth 1-2 levels and the chance to roll some nice upgrades. + all the shop scumming for spell books. Point being, I would be in a much better position to handle him than you are now typically. I took him down really easily in my game thanks to what was a much better resource position relatively speaking. There are a lot of things in this game that require you to take them in stages. Sometimes it’s hidden areas in a dungeon that have much tougher monsters. Sometimes it’s that pesky boss. Ba’Dong is extra tough because all demons are a pain. I don’t recall hard CC working on them you have no doubt discovered.
As for sleep, anything that allows you to “turn off” enemy archers and mages for a few turns is a godsend. As a general rule, humanoids (from actual humans to orcs), and *men (uh, you’ll meet plenty of flavors later :D ) are generally susceptible to all of it. Although more powerful creatures can be more resistant . In terms of things being too resistant, you generally won’t have too many problems in the top part of the initial island, or even down part of the east side though. Bosses are often trickier, of course.
Oh, to add: a ring of quickness (+1 movement minimum) will run you ~3.5k gold. Keep that in mind if you want to keep money on hand while looking for one (Galatako is the most likely place to find one for a very long time, merchant wise). Your best barterer will affect that of course. I think I payed 3600 and change, but don’t have anyone with good barter (probably no better than 35-40).