After like 20+ orbs spent trying various strats, I've finally beaten Europa and finished rank S-5 colloseum. What a pain in the ass fight, just wow. Some misc tips from my many attempts. All of Europa's special attacks are lightning elemental, and Siren gives -50% lightning weakness, which is basically a death sentence if that unit is targetted by wave cannon, so unequip Siren before the fight. The item "Leather Shield" has a built-in +20% lightning resist, put this on as many units as possible that can wear shields. If you have a unit with built-in "Draw Attacks" trait, it's a good idea to just make them a tank bot by guarding them every round to soak up hits.
Status effects do work on the boss, but I found that using pure status skills (like just casting "Silence" or "Blind") isn't worth it because the boss has heavy status resistance, and a failed status will just waste that units turn for nothing. On the other hand, using attacks that have a built-in status are great, such as Blade Bash or Silence Blade. Speaking of Silence Blade, if you have a magic key from crafting, or you bought one with lapis, you can get the recipe to craft this skill from Latius Woods. It's a really great skill to stick on Rain and Lasswell and let them spam it for most of the fight.
I strongly recommend at least two aoe healers for this encounter. If your healers are red mage types with both heals and nukes, you may want to pass on the nuking to preserve MP for only healing. The boss has a ton of hp and is poison immune, so it's going to be a long drawn out encounter. My final tip is be ready to try the boss multiple times before winning. Sometimes RNG will just fuck you when he decides to wave cannon, plasma bolt, and critical attack the same unit in the same round, which will kill even a tanky 5* unit.
If you have Vaan for full break, or a unit that can cast Bar-Thundara for AoE lightning resist, you will have an easier time than I did!
