this is so wrong. It's been stated (can't post scans since am using my phone) that the power of squad zero is greater that the combined power of the gotsei 13. Squad zero is the personal bodyguard of the SK so I don't know how someone not in that group can be stronger than anyone of them. Don't get me wrong,Yamamoto is scary and as soon as he died,squad zero had to get involved but the members of squad zero are above him. Juha didn't even fight Yamamoto but his well involved with squad zero. That has to tell you something
The quote is along the lines of their combined strength is greater than the Gotei 13. Which at first sounds intimidating, but really isn't.
The Gotei 13 is a joke honestly. Only the Captains and Lieutenants can do anything... And even then it's really a handful of captains that have great power.
Based on feats, Yamamoto can solo the Gotei 13 alone. No one can stop Zanka no Tachi.
The Zero division are people that seem to be at a level a bit below Yama-Ji. Ichibei is probably on par with Yamamoto, but still loses because of Yama's bankai advantage.
All it takes is a couple of them ganging up on Yamamoto to stall him, while the remaining 2/3 solo the G-13.
Unohana, Shunsui, and a healthy Ukitake would be the only people with the brains AND brawns to give them a legitimate fight....but they won't last.
So really the Gotei 13 are a small handful of capable fighters and a lot of cloth to cut through. Yamamoto, Unohana, Shunsui, Ukitake, Byakuya, Zaraki, and a prepared Mayuri are basically the core of the Gotei 13.
Byakuya doesn't have the fire power.
Mayuri doesn't have the fire power.
Zaraki doesn't have the brains.
Shunsui, Ukitake, and Unohana have both...but are still out played by hype (0 Squad is likely ranged from them to Yamamoto which is a decent gap).
So yeah it makes sense they can defeat the G-13. All you have to do is stall Yamamoto, and then mopping the trash that is everyone else.