I can usually manage to kill Ike in one go with my blue mages or Kagero, but my most reliable Ike-killers are actually Ninian and Marth. Ninian’s usually-garbage stats seem tailor-made to take on Ike, while my Marth regains 10 HP on every second and third round (from renewal and his sword respectively), so he can usually beat Ike through attrition. The teams that I struggle with are the ones that run both Camus and Xander with vantage.
For some reason, though, I can never work myself up to actually wanting to do Arena Assault. I think it’s because if things go wrong, bailing out and immediately retrying is usually an option, except for that one mode. Oh well! I’m happy getting my middling rewards for muddling through it once or twice a week.