User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170726154553/@comment-10330822-20170811004644

I'll agree that he probably hasn't mastered CxC, yet. However, he's probably closer than he was in Volume 20-21. We really don't know what training he did between his promotion to High-Class and the beginning of the tournament, it was stated to have been a few months. For all we know he could have mastered it and it was never mentioned, still I feel his CxC would be far more OP if he had, and he probably could have beaten Dulio without even messing with his partial-arm DxD transformation.

The DxD form is supposed to be "unstable" because Ophis and Lilith aren't "cooperating". Whatever that was supposed to mean. It could mean that they need to fuse for Issei to use it properly, or they just need to be in-sync. Either way, Ajuka implied that Issei's mastery of DxD isn't something he can do with just stamina training alone. Ophis and Lilith need to have an active role in it as well.

As for AxA and ExE, we have no indicator that his mastery of DxD is even related to these forms. Sure, the GR body helped him use Ophis's power, but mastery of the GR body never was stated to be a factor to master DxD, unless I'm mistaken.

ExE, on the other hand, if the trend fits, with me staked on his DxD and other previous forms, just like DxD and CxC were on his CxC and BxB respectively when he activated them.