That was the general idea of the latter suggestion. I was thinking training officer for aurors, seeing as I always imagined American mages being ahead of the curve in terms of large-scale magical warfare [because we know the british are fucking awful, at the very least]. As for what his weakness is, I don't know exactly what you mean, but he is a specialist. Soldiers are not known for their vast academic knowledge, for their ability to train magical creatures or maintain advanced magical machinery. Specialists tend to get fucked over when they are doing things outside of their skillset. He is pretty bad at potions [he had his grenades issued by Uncle Sam], poor with magical creatures [he never liked the 1st Cavalry nancies anyways], not book smart nor read up on any history outside military matters. Bad at boormstick flying, a tad jingoistic, fairly unsubtle etc.

If you're looking for serious combat drawbacks, I've got few ideas as to what it would be. Veteran soldiers do not tend to have glaring faults in their combat capabilities: they either died because of them before they rose in rank, or sorted the faults out during their careers. I could come up with something, but I think if serious combat weaknesses are required to play a combatant then I'll find some other niche.

EDIT: Just realized this sounded a little passive aggressive. This is what I get for posting before my first cup of coffee. Sorry bout that