Aizen couldn't stay on the battlefield with Yama if he went all out full power, hell the entire SS would be destroyed at his whim obviously. So in my opinon even after Wonderwiess stole some of Yama's power, that was only in Shikai. He could've gone SS 2 where he couldn't be touched, SS 3 South where he called his minions, and he's not only untouchable, they'ed or the flames and heat would eventually get Aizen.
Show Me How Much You Have Evolved...(As Tian walks toward an Army he's been waiting eons for).
I speak 4 languages...
English, Profanity, Sarcasm, and Real Shit.