I Once Had A similar situation except I was the DM and the wizard was evocation focused, but got leadership and a wizard for a cohort. The player was actually quite new and needed my help a bit in making a 14th lv wizard in the first place which was fine with me, even so he was a very intelligent person and had alot of time before we played so by the time we started his character had been months in the making and it was covered in more sticky notes with page numbers and note to remember stuff then I've ever seen before. He did his wizardry homework.
Anyway we've played for a bit the start was a bit roleplay heavy as my games tend to be but eventually session 3 or so we get into a big combat with lots of enemies to contend with (we were 6 14th lv 1 mythic tier characters I wanted us to maybe struggle) yet his amazingly prepared spell list ruined nearly every enemy in 2 spells. At first I was slightly annoyed with the character until I realized my anger was completely unwarranted as he had done absolutely nothing wrong, our group was also not as upset by his long turns which did consist mostly of looking up spells, but were obviously a little upset to have not been able to contribute to the fighting as everything died as they got to it.
I decided I wouldn't penalize the Wizard however and took the blame on myself as the DM and learned that I would just have to contend with the Wizard as best as i could. This started easy enough with the simple fact of him blowing stuff up so nicely that he made an obvious target of himself, therefore it was easy for me to pick him out and target him with monsters and such without it being unfair. I also made sure any npc caster or monster with spells had dispel magic and the like, if they didn't have it prepared or know I would change that as fairly as possible so they did. I was playing forgotten realms so the magic was already pretty prominent, i didn't see the harm in people frequently preparing to fight magic. This helped against alot of the AOE spells that could last up to 14 rounds and such and i think could even help to send monster that have been summoned away, that last bit I'm not quite sure on the accuracy of tho. Finally, and this was heavily house ruled by myself I guess you would say, but I just secretly bumped most enemies stats a bit to either make it so he wasn't 1 hitting every single enemy on the field or so they actually had a chance to pass a save.
All in all I guess I'm just saying that you should maybe talk to your DM, and party, and maybe help him get the handle of what he has gotten himself into with this game since it seems as if you may know a bit more about the high level stuff then him. Also whoever said to maybe save the horde for the boss has a good idea too, just stick with your cohort and crazy int. Item until you need all those big guns, people might appreciate them more if you pull them all out when you guys are about to die rather then when it was just overkill. Now realizing I'm writing a book sorry everybody lol