Overkill is not a concept that exists in Yamamoto's mind.
Yamamoto brings out his insta-kill sword and untouchable suit of flaming armor. Now that his sword is gone, Bach results to firing an arrow but Yamamoto easily blocks it. Yamamoto charges but Bach sets up a giant trap spell to catch Yamamoto, but Yamamoto calls the South portion of his Zanka no Tachi bankai and summons a army of skeletons.
The old man is showing his hand, and at least for the moment Bach looks intimidated. Apparently the insta-kill sword and untouchable suit of flaming armor weren't enough, Yamamoto summons a skeleton army. And lets not forget that he probably has a North component to his bankai which probably drops the sun on the world or something along those lines. Makes you wonder how Bach can possibly defeat this guy, but of course Bach will somehow beat the old man. Aizen beat the old man with genjutsu. What's Bach gonna do?