So, I'm not sure if anyone noticed this or not, but the monsters do, in fact, level up like they did in the past. They just don't display anything in terms of leveling up while its happening. I haven't tested it further to discern whether or not the level stays permanent after its dominated, but that's next on my agenda.
In past builds when leveling them up was a thing you could see their max HP increase each time they leveled. That doesn't happen with this build which makes me think it might not work. But trying to kill one that has been leveling up for a while seemed to have super high armor rating as my attacks did very little to his HP bar.
Edit: Correction, monsters do increase their max HP when leveling. I likely was paying more attention to a higher level monster which doesn't level very fast due to their already higher level.
So tried spawning 10 Ogres and the lower levels (11, 12, 18) leveled the fastest as expected, but they seemed to continue to level at the same rate which means lower level monsters are the best to do this with. I waited until a level 11 and a level 18 both were above 5,000 HP and their attacks were dealing damage over 3,000. After I dominated them one was lvl 123 and other was level 133. Ogres are capped at 60 from what I can tell and these guys were hitting hard enough to one shot me with just under 200 armor.
I'm going to try some Drakes now, but the concept is proven to work much like it did in past builds. The monsters also seem to greatly improve armor likely from taking hits from other monsters while they are all leveling. The weakest one was a level 42 that didn't really increase HP much (~2500), but it still took maybe 40 arrows in the head to kill. Killed off the rest with greatsword dealing 150+ damage and it only did maybe 70-80 damage per attack tops sometimes under 50. So their armor gets crazy when they level as well.