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Your Zelgius has 5 attack and 3 resistance in exchange for 1 speed and 2 HP, he's just a way stronger unit.

As for seals, it depends, but Iote's Shield won't do much for her.

* Should I use Black knight or a +atk -def zelgius?

Same difference. Use Zelgius if you are trying to get extra scores in Arena.

* Should I use guidance or iote shield seal on Cordelia?

Guidance.

Iote Shield or not, Cordelia is an easy OHKO. (Though it goes both ways.)