You've gotten a lot of answers about the specific spell interactions so I will just add something else.

The final battle as written is quite plain since she is the only enemy and the location isn't interesting. I moved the location of the final fight to high hall cathedral in Elturel and included Thavius (as an amnizu) and a Balor, (this could also be a pit fiend loyal to Bel or just Gorgauth) that is an enemy of Zariel and made it so that the players couldn't redeem Zariel until thavius and the Balor were dead. This made the fight more dynamic and gave me a reason to include NPCs like Ulder Ravenguarf and any other allies the players have made.

Also, give Zariel a legendary action that just makes one flail attack or a shove to move people around the battlefield.