Were you wearing the same armor and have the same resistances?
Some bosses are more resistant to some things. Also keep in mind that lvl 26 - lvl 31 is an EXTREME difference. That's five levels. That's also 20 attribute points, and at least 5 skill points. Also a big difference in gear.
It's much much more than just levels.
And wisdom is not a good investment. Not using 10 skill points to win a level is just silly. I'll battle you at one level less when we're both with a melee build and summon demon at level 10, it won't be a contest.
Always fun to see mages expect to one-shot people AND be as resistant to damage as a warrior.
It's called L2P.
