Since he has those 13 years of experience he probably knows more spells than any other student and probably even the teachers. Plus that was his main expertise, since he could analyze and seal a dragon's magic in a very short amount of time, which is something even a 7 tiered magician couldn't do that easily or quickly. So even if he doesn't have the OP mana and artifacts anymore he can probably reach that again much faster, and until then he'll have his knowledge to take advantage of all the spells weak points. If he's sensible then he would also help his friends reach their power pre-reincarnation as fast as he/they can to make it much easier for them to prevent the extinction or near extinction of the human race.