Chained Echoes Boss 21: Forgotten Matthye
5Here we are the big roadblock for a lot of people. For those struggling my advice is lower the difficulty... but for those of you that don't want to do that I highly recommend getting Tomke before starting this mission so you have an additional meatshield. Otherwise Amalia is a lot tankier than she may appear to be.
Of course a real kick to the face is that his stealable item is a weapon for Tomke, that you could already get before this. A lot of Matthye's attacks honestly don't hit that hard the issue is he gets to act twice and that is going to overheat your team very quickly which is where his damage becomes so dangerous. On top of that he can lock out some of your commands or attack types in phase 2, I don't think he can lock out healing but he can lock out items.
My best advice is as far as damaging him play the elemental game by giving him weaknesses and exploiting the crap out of them with Lenne. Ba'Thraz can also hit his weaknesses naturally and human slayer may be a good passive skill if you have the points lying around for it.
Also use your Ultra move in phase 1. It'll be a a good chunk of damage and will refill by phase 2. When in phase 2 I'd recommend hoarding your ultra move for either Glenn or Amalia. Glenn can hit for good damage and inflict a ton of debuffs even if debuffs are locked out and Amalia can fully heal and revive the party. If you're healthy but need TP desperately then Victor is also quite good while Sienna and Ba'Thraz do good burst damage. Don't use Robb's Ultra and only use Lenne's as a last resort since her elemental thrusts can hit for good damage anyways.
Really this fight comes down to managing your HP, TP, and Overdrive constantly and given his lockout mechanic you probably need a bit of luck as well.
Either way don't get discouraged this is an extremely difficult fight and there's a reason the game gives you a lot of 6 star weapons before this. Again if it's just too tough then don't be ashamed of lowering the difficulty I had to do that for the monk trial.