Wow, this doesn't match up with my experiences at all. I'm just at the point of working my way through the vicious dungeons for the first time (and have been for weeks, I guess). Maybe it has to do with with items I unlocked in which order, or which strategies I thought of first (I didn't start reading the forums until recently), but here are my thoughts:
I tried out various vicious dungeons when I ran out of hard dungeon quests. 50 death protections got me scared and alone in a room with 3 bosses blew my mind.
1) I unlocked the Half-Dragon first of the monster classes, Dragon Isles was also my first vicious win. I don't believe I had any one strategy that made it click: my wins so far are with warlord, assassin, sorceror.
2) Was so confused about how to deal with the other vicious dungeons I went to the forums for the first time to look up stuff, found I was missing some items, so I went back and got those.
3) but it was the alchemist scroll that finally got me through Naga City. I used this strategy with 3 different classes:rogue, thief, bloodmage.
4) then I went back to Namtar's, more confident now with the alchemist scroll, and beat it first time with the rogue. Haven't gone back.
5)I still couldn't seem to take down 50 death protections. I tried out different things, eventually I found prepping Yendor +knockback worked. I used that for all three: Half-dragon, sorceror, beserker.
And I haven't managed Demonic Library yet or GaanTalet yet.
In terms of enjoyment, I liked dragon isles originally because I could manage it, But Naga City wins out now because it's just such an everything-must-go god-converting fiasco. forced me to start playing around with the gods more, where before I was generally picking one god and sticking with it.