There was the 4 Fire Giants as well, and the Paladin got into a one on one with Bel(dragonborn hates Giants). I didn't end up using Meteor Swarm or Imprisonment, just Legendary Action fireballs and 3 attacksActually, I'm surprised they had problems with Bel with that much magic. At the end of our Avernus campaign our 13th level characters with much less in terms of magic (except for our paladin) were able to defeat him. Tough battle, but with buffing like Heroes Feast against his fear, not impossible. We did have more than four PCs though.