+Res/-Atk Zelgius or Black Knight?

Zelgius for long term investment and Black Knight for immediate payoff. Black Knight is gated by limited merges, has a naturally lower BST for arena scoring. Comparing the two, your Black Knight will have +2/+1/+1/-3/-6 compared to your Zelgius. Overall less bulky, with only +1/+1 in ATK and SPD which could make a difference but generally won't since Black Luna will usually overkill your enemies. Besides if you get another Zelgius later you could always just merge into the better one. Up to you.

Well, here's the thing, you can give him the Quick Riposte seal or trade Warp Powder for Vengeful Fighter, you've doubled his damage output, and if you're keeping his native Fierce Stance that's +6 atk to both atks when the enemy initiates. And then he'll be one hell of a Sword Duelist + Anti Green Mage killer.

Finally, pair him up with Hector, who has Goad Armor, and that Atk IV won't matter.

When you invest the right skills and build the right team support, the IV's just don't matter XD So, it really depends on how fantastic that Black Knight is you have. Good luck!