I would give you some advice, but honestly it was the first class I won with so I don't remember now what choices I made. For me, my Achilles heel in Dungeon Run was Paladin. I'd build for Recruit buffing, or I'd try to build for big minions as these were the two suggested strategies. But no matter what I'd get hit with a final boss that countered my strategy. Until one day a few months ago I randomly decided to try it again and actually cruised through all 8 bosses.
I actually didn't keep track of how many bosses I defeated before completing my first run with each class. Although I did record how many runs it took me until I got my first 8-win run. I've also kept track of how many total runs I've done so far with each class, and how many 8-win runs I have.
Druid: Took me 5 runs to get get my first 8-win run; (Right now, after 25 runs, I've completed it just that 1 time).