Should I kill Ishtar for Ophelia?

My suggestion is to play around with Ophelia first and then decide if she actually needs additional power.

Well they are both blue mages so you're not really losing on that front just making one better. What ivs are they? Can hold off and play a bit since they do have different styles (high attack aoe nuke vs speedy single target nuke)

This just destroyed my soul, Ishtar is a really good unit and she has saved me so many times and I prefer her over Ophelia since she doesnt require as much setup.

I also have Ophelia with Special Spiral without Odd atk and she stills anihilates every map, I'd recommend keeping them both or at least wait until we get all the summon tickets, maybe you can pull one last Lewyn from there.

Ishtaar is so hard to come by and has better bulk and power.

I wouldn't, it's a great skill but you can get 6 offense other ways

I'd be much more ready to feed someone ishtar as opposed to lewyn. That inverted desperation on his weapon is awesome (but then again my ishtar is atk bane so maybe I undervalue her lol)

I don't think having odd attack wave on her is a huge deal, though. You could always just put tactics on a teammate or whatever. Also, for AR you wouldn't even probably want her getting buffs at all, since shes going to be constantly panic'd with her low hp pool. And since she's not really viable for arena use with her godawful bst, I mean thats like hyper-optimization for what'll probably amount to pve content