I am not being condescending but most games are easier in the first stage than the second stage. So upping the difficulty in the first stage would not be the same as upping the difficulty in the second stage.
If you can do MP6 in act 1 maybe act 2 needs to be MP3-4 or something. The Monsters are a lot more annoying but act 4 monster are worse with subjugators and 1 shot mallet lords. I know early on in the game phase beasts from act 3 were the most raged about enemy. So it is a natural progression making monsters have more abilities or differences as you progress through the levels.
Either way if you just want your 5 stacks you can get the pretty easily without going near wasps and the like. Oasis has heaps of mobs and they are usually disappearing guys (straight melee pretty much) Those hammer guys are melee. The spinning warden type monsters can be annoying. But should be 5 in there and then do warden. You only want the key you don't want to farm act 2 cos farming you can do act 1 like you said. So it's all about 5 NV and warden.