How to improve my Zephiel

He's one of the main units in my armor team. I want to maximise his defensive potential, so change his weapon isn't are part of my plans. Plus, I'm F2P player, so it's difficult to me find DC fodders.

I've thought on Ignis, Brazen At/Def (I have one -At W.Chrom) or another seal. Which do you recommend me? Thanks!!