Good Build for Lukas.

Hmmm... With a +Res nature and Berkut's Lance, Lukas manages to hit 31 Res in the enemy phase, which is pretty decent. The only problem is that his low Spd leaves him prone to being doubled by many units.

As for the A skill, Fury could be an option, but I'd recommend it only if you want to run a combat medic alongside him, since it could eat away at his health fairly quickly after multiple fights. Fortress Def/Res are solid options, if you're OK with the - 3 Atk, or sacrificing Innes (but I wouldn't recommend doing that if you do have one). If you're willing to sacrifice/have a W!Chrom available for fodder, Brazen Atk/Def is a good option. I'm honestly not sure what to choose here since you're running a mixed build, so my apologies for that.

I'm at a loss for what to choose for the special, but I think Ignis might be the better option in this scenario, especially if you're going to use the QR seal on him.

For my Lukas, I went for the full defense route with Slaying Lance (+Def), and he ended up capping out at 50 HP and 50 Def, and 46 Atk with Fortress Def in his A slot. He's been an incredible wall for me so far, and he's able to do an additional 40 damage with his Ignis procs. I usually run him with the Deflect Magic seal depending on the team he's on, and he does fine. But it's ultimately up to you to choose your build.

If you're going to want him play the role of mixed tank, he needs Distant Defense. You've got some options here. I'll list two builds I believe will work for you, though I'd really like to know his IVs.

Lukas Aether Build
Aether
Distant Counter/Close Defense
Quick Riposte
Distant Defense Seal

These are the requirements. DC is really needed to make this set go from good to amazing. However, that is an extremely rare A Slot, so I can understand not having access. That said, I suggest then going for Close Defense over Distant Defense simply because his preferred combat is against melee units.

Lukas Ignis Build
Ignis
Distant Defense
Vantage
Quick Riposte Seal

Thanks to our newest Seal, QR+V will be everywhere, and Lukas will make great use of it. Because our Seal is used for Quick Riposte, our A Skill MUST be Distant Defense. Having combat with a melee unit will have Ignis at the ready for the next combat.

There are other ways of making this work, but these are the two I'd personally go with.

Don't bother with resistance on Lukas. Just don't. Capitalize on his high def or atk.

Take my Lukas for example.

Slaying Lance has good might and reduces the special cooldown. Take Defense refinement as neither Speed nor Resistance is worth saving and Might refinement is never worth it on any unit.

His default Fortress Def is great, as his attack is already high enough and with 50 defense, when Ignis procs, he'll do at least 40 damage, killing all but the most tanky. Plus, very few things will hurt, let alone scratch, him with defense that high, and we only want him in melee combat barring dragons anyway.

Quick Riposte allows Lukas to strike twice and proc Ignis to deal huge, and often lethal, damage to the for. You can use Quick Riposte 2 if you don't have a 5* with QR3.

Threaten Atk is here to lower enemy attack. There's already little hope for the enemy, but this to ensure that their weapons break against you, at least until Wallace comes. Hell, he'll probably even be proud of you. But, Threaten Def can be taken if you want more damage.

If you really don't like this build, there is a Brave Lance set for Lukas, using Death Blow, Drag Back, Threaten Def and Moonbow with Quickened Pulse. And there is a Brash Assault seal and Desperation set that might be fun.

What do you think?