I figured it all out.
This is just like Diablo. In Diablo, you have different categories of enemies you face: Demons, Undead, and Creatures. Demons and Undead are fairly self-explanatory. The other creatures are things like angry goatmen, giant mosquitoes, yetis, water serpents, etc.
Therefore, the Daimons in BM are the Diablo version of demons, that encompass a variety, just like in Diablo, to separate the foot soldiers from the bosses like Mephisto and Baal. The Undead in BM are just like the Undead in Diablo. Actually it's lols cuz they got played pretty much like the Lich King's dudes in WoW in 4th Inv. Ok, so it's not Diablo. It's still Blizzard.
And the Monsters were everything else, so you can't go wrong there.
This means I just figured out everything, lol.
You just need to party up, get a good tank, have a healer, and do some nasty DPS to the daimons. Then get Overlord's soulstone and reassemble the Hellforge Hammer. After that, you shatter his soulstone on the Hellforge or something. Problem solved.
Although a difficulty arises because we can't really safely beat the 5th Invasion unless one of us knows spoilers for Diablo 3. lol. But that comes out in March. So let's try to stall the Invasion until then. lololololololololololol
