Another route through this map.
Treachery (RA Lucina's assist) with a couple of buffs and Hexblade (Mauvier) improve our damage. Could have gone for more damage if needed by including Def/Res buffs into the team.
Null Panic (Mauvier) and Neutralise Penalties (Hortensia) helps with our damage as well as tankiness.
Hortensia throws out a lot of debuffs which might not be relevant against the two mythics but probably helps gimp a lot of the mooks, who are designed to be quite resilient.
The idea behind the tanky enemies is presumably to make it difficult for the player to reduce enemy density, thereby making it difficult for the player to deactivate Eikthyrnir's weapon effects.
On the penultimate turn, you see me swap Lucia down then swap up again — this is because I need to burn her action so she shakes off [Deep Wounds] and can thus receive healing. Otherwise I'd have to retreat and this would be slower.
Breath of Life 4 works on Lucia if she has it equipped herself, though her defense not being high, her healing is often not stellar.
I ran Bulwark to keep the supports safe, if you run Gambit you will deal better damage but you'll need to adopt a different formation. You likely won't need Treachery in that case, so bring whoever you want.