[Suzerain of Sheol]

I gave it another test after the fixes and I beat him pretty easily. Just make sure to use the few buffs you have at that point, and if it gets tough you can always have Tristan switch to dedicated healing since he's quicker than Amilia (and should be faster than the boss if he's at least level 6.)

Oh, and I don't know if you like poking around, but there's a Piety Scripture in Orec's house that can boost Tristan's magic damage for using Wildfire against Liukaeryos. Actually, it's a good idea to check pretty much everything wherever you can, as I have stuff hidden all over.