I am gonna hazard a guess that your damage is way higher than Yoshi's. Yoshi solos mp10 ubers all the time on his wiz. Are you using your wiz to do solo ubers?
I used my monk, I can play a bit more sloppy with him without dying.
I can get my wizard up to around 230k DPS but I made a Force Armor build so my health is only 15k so I can take 4 hits before death. Which for the most part is fine I can breeze through all MP8 Ubers except for Kool and Breaker....I really have to watch how I play or I'll get tornado'd....Which is almost an instant kill.
So I use my tank monk with 5 % life leach and 220 Life per Spirit spent and watch TV while killing them.