User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-77.182.125.30-20180521144211/@comment-2602:306:CD9A:C860:2CE1:7CE7:5D34:69E3-20180523075140

Vali hasn't mastered his EJOD yet either and his DxD L shaves off his life force everytime he use it so he needs to master that as well. The only thing Issei needs to regain his True DxD G to have Ophis and Lilith resonate first and there was already signs of that in Vol 23 when Issei notice Ophis and Lilith chanting something together. It's true Issei needs to fully master CxC but that will take time just like it will take time for Vali to master EJOD but both can already use those forms without difficulty.