Board Thread:General Discussion/@comment-39156026-20190425022331/@comment-5398287-20200328013554

In her own words, she had to kill and revive him over and over again in order to bring out his full power and allow him to finally hear his Zanpakutō, which could only be done in a battle to the death. Hell, he didn't even INTEND to kill her and wanted to be able to fight her again, but she let him do it once he'd finally lost all his mental restraints because it was what she always wanted. So yeah, there's no "if she trained him well enough", this was the only way to prevent the war from being ended early by someone with the power to literally warp reality - even if she didn't know exactly that the enemy had such a combatant, she knew Kenpachi would be needed to take on the heaviest hitters, especially as a Special War Power.