Each wave has 3 enemies, and the next wave only shows up after the current 3 are defeated.
Take advantage of enemies' status ailment weakness. Dario alone can already inflict poison, silence, stop, or stun. Raising his faith stat can help increase the chance of it working, but will make him take increased magic damage, which can be dangerous.
Mia is a pretty good unit to bring for this fight. She's an R rarity unit, so it's easier to get her shards to reach lvl 99, and she can also inflict charm by using Steal Heart, which can be quite game changing. There's Steal Time from her main job sub job as well.