Vanguard, my great general char managed to kill one of the bosses in there. It's a pretty tough place, but I think if you manage to clear the other dungeons quick enough then it's possible to have enough remaining months to clear it at leisure.
Skeletor, the items are grouped into charm, combat or magic bomuses - it should be obvious which are which. One notable item is the glove in the first dungeon, which restores HP if you kill a monster with a combat special move.
I'm not sure what you mean by the bonus/malus of the classes. I will say though that the game doesn't really reward multi-classing - go all out in one area, and try your best to train this area in dungeons with kills (kill with magic, special moves or charmed monsters killing for you). Train athletics a lot for extra hp, and be sure to rest up a lot towards the end of the month so you have enough stamina for adventuring.
