For summoning skill, I'd reccommend having it capped. And for the items, I'd reccommend Ethers and Hi-potions, lol. You might not need the potions though, but I'd suggest keeping them just in case. And, he should take his blackmage to around... well, I'll say 65, cuz I don't think he'd be much use to any Fenrir parties. >< But yeah, unless you have a bunch of high lvl friends who wouldnt mind fighting Fenrir for him, I'd suggest you wait for him to be higher lvl, so he can join Avatar parties.
Also, this is a really good site for avatar strategies and stuff: summoner.info. I looked there when trying to help my in game {husband} think up of strategies to kill the avatars.
Good luck to you guys!