More unfavourable IV advice

Even if you don't want to put him on your main arena team, I would personally still build him for arena assault. -def is his best bane and while he doesn't have +atk or spd, +hp is fine. He can be really useful as an anti-mage unit.

I'd keep him. At least he's not -atk or -spd, and those ivs let him take on mages even better. Id keep his Nidhogg anti-magic niche, and build him until you get a more favorable nature to merge him into.

I don't understand how those are unfavorable IVs. He dies to almost any physical attacker anyway and that just gives him additional magic bulk.

Mine is -Atk/+HP, and he's still an amazing mage baiter for Arena Assault. Keep Nidhogg (there are better BB+ users anyway) Fortress Res, and Cancel Affinity and he'll be able to bait and kill the vast majority of mages (even most Quickbow Reinhardts can't chew through his potential 36-42 Res and 39 HP).