User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-82.160.23.173-20190112172331/@comment-10330822-20190122063307

Well, to be fair the Illegal Move and CxC were something that was unlocked by his willpower, as substitutes for the Juggernaut Drive, but then had to be mastered afterwards. Issei is in the habit of unlocking modes, and then mastering them. He doesn't train for the next mode, it gets better at the modes he had and a new move unlocks as a result. This is also what has happened with DxD. The problem is the scale of the power is so great in DxD Issei couldn't just simply train around it. He's needed a lot of outside help, i.e. Ophis and Indra, to really get a handle on the form and make it effective in battle.