Just got Wizard in my game, so here is very limited, but fresh (early game) experience (sry. for poor english):

At this moment his damage is very low in regular fights for 2 reasons: 1st - you need to build debuffs on target(s) to deal damage, which is a weak mechanic vs normal enemies, since you want to focus fire most dangerous ones asap, not waste several turns. Just look on marks - they provide much stronger boost (short-term) in damage with only 1 check vs 2 wizard checks (attack vs attack + debuff check) and can be set by heroes over than DD. And still many players consider marks lacking in regular fights. 2 - his base damage is very low, for some reason his weapon progression is much weaker early. Give him ~BH damage and progression plz, he really need it, especially early.
So far i didnt put him vs bosses in slogfest type of bossfights, but i have a feeling what in this scenario such mechanics will turn in opposite direction - he will stack debuff to absurd lvl and nuke boss with absurd damage. Which is a bad thing too - hero need to be good in most scenarios, not "suck in 99%, op in 1%"

Did you consider to move his damage from debuff to self-buff with finite duration? It will allow to progress in normal fights much better and will lower his power in slow bossfights.
Ofc its a pity to abandone such work of coding, but imho balance-wise it seems like much better mechanics, and MUCH more reliable - it work vs groups of normal enemies, where you cannot stack debuff really well, its work vs enemies with high debuff resist, and still quite powerful vs big fat enemies, but without such lvl of "abuse power".

Almost forgot - SPARK DAZE effect didnt work really really well, since SPD rolls once when turn start, and if you use it on enemy what didnt made an action yet - it will not put him behind in round queue. Enemy just act normal and waste this debuff. So this effect is limited to only vs enemies what already act. Maybe just made it ~10 spd debuff for 2 turns? Or this is intended?