Legendary Dimitri

This is a typical AoE burst build for initiating a fight and weakening enemies with damage and debuffs, mostly using Dimitri's base kit to his advantage. With this build, you don't want to engage much in enemy phase and instead sweep the enemy in player phase, starting with Dimitri initiating.
[ Boons: +ATK | Banes: -RES ]
The crux of the build is sweeping up an enemy with your AoE special, and thus +ATK is really the only boon you want. For banes, while -RES hurts his magic bulk, you're not really aiming to engage on enemy phase, so bulk matters less. -RES is simply chosen since it's already pretty bad.
[ Weapon: Areadbhar ]
We really just want Areadbhar for it's high attack and slaying effect. The other effects are of course great still for general scenarios.
[ Special: Blazing/Growing Wind, Thunder, Light, Flame ]
The AoE special. What type of AoE is up to preference and situation, running his kit Blazing Wind is completely fine.
[ A skill: Sturdy Impact/Death Blow 3 or 4/Life and Death 3 or 4 ]
A skill serves to increase damage output. Any a skill that gives +atk is good, but if you're looking to maximize AoE damage then choosing an A skill that increases visible atk like Life and Death or Fury is better. Those skills however makes his tanking a little worse, especially -RES + Life and Death making fast mages a serious threat. Keep that in mind.
[ B skill: Atrocity ]
The extra damage is great, but what really puts it up is the Rainbow Smoke + Pulse Smoke afterwards, it makes it so initiating combats with him allows your other units to sweep up nearby enemies much easier.
[ C skill: Time's Pulse ]
The C skill and Seal work together here to instantaneously charge your AoE special after combat without needing extra enemy phase combat. The charge is set to 3 from Areadbhar, Time's Pulse gives -1 charge at turn start, and after unleashing, with Dimitri's high attack and Heavy Blade, he gains -2 during combat (assuming Dimitri OHKO'd the enemy, which with his sky high attack and an AoE special, he should've). The total is -3, and he's ready for the next player phase. The problem is the first combat, since he starts the map with 2 charge. There are a number of ways to remedy this, such as Infantry Pulses or Supporting Velouria, whatever floats your boat. If you don't have Time's Pulse fodder or Heavy Blade seal, then the alternatives is running an attack increasing skill, such as Rouses/Oaths, or Joint Drive or Hone Atk and such, but these aren't as good as they don't benefit special activation other than adding on some more oomph to an already hard hitting oomph.
[ Seal: Heavy Blade ]
See above.

Feeding Dimitri a spare Thrasir makes him a TERRIFYING threat. The duo rally can be anything you want. I just gave him Spd/Def+ 'cause it's what I had on hand and was gonna use him in Arena. Here, he charges Galeforce in 2 hits, takes less damage, has bonus true damage based on his atk and then debuffs ALL stats, Special charge and panics. Spd/Res solo is to help him *attempt* to take a magical hit if he's alone. You could go Atk/Spd if you want more offense, but this way he's sitting at 24 Res and probably 40% DR. Against some red and blue threats, that's not bad odds.

This is a standard galeforce build for Legendary Dimitri. The sacred seal could also be changed to Spd/Res Solo 3, but since L!Dimitri Res is awful, it wouldn't make sense to use it instead of using Atk/Spd Solo 3. Time's Pulse and Drive skills could be used instead of Odd Tempest if one plans on using Legendary Azura or Duo Ephraim along with Dimitri.