Lukas

Lukas is already an enemy phase unit, so lets just make him an even more enemy phase unit. These skills are hard to come across so use this build if you are actually going to be using Lukas a lot.
Berkut's Lance+ (+Res or +Def) or Barrier Lance+ (+Res or +Def) depending on which ever one you like to use. I believe +Res is preferred to make him a mixed defense unit.
It doesn't matter what positional or rally skill you use but I use swap.
Ignis for killing this enemy because his defense is really high.
This may sound weird, I dont know, but steady breath for +4 defense AND +1 special cooldown per enemy attack, since this is an enemy phase skill this would go well with what Lukas is doing. This goes well with Ignis and Special Spiral 3 because when Ignis activates, it will probably kill the enemy and Special Spiral 3 will activate leaving him with only a 2 cooldown count. Then when the enemy attacks him again, he will already have Ignis ready (from steady breath) for another kill. Though this does become not that good against units that have those skills that slow down someones special cooldown.
Special Spiral 3 was explained with Steady Breath.
Def/Res Oath 3, or maybe rouse Def/Res 3, or even a defensive wave skill just in case you don't have any of those skills. This will add more defenses on him to make him even bulkier.
The seal can be any defensive skill but preferrably one that raises his resistance.
I actually don't run this exact build as I don't have def/res oath 3 and I use +def version of Berkut's lance, but it still works for me probably because I have summoner support with Lukas. I don't even know if this is a good build.

Vanguard+ (+Def): Gives +4 def to base stats and +7 if the foe initiates the attack.
this weapon alone brings Lukas his Def to 52

Swap: Swap is handy to get other allies out of enemies range.

Steady Breath: Next to adding +4 to Def when the apponnent attacks, it also makes it so the cooldown for Ignis is charged with just 2 attacks.

Renewal 3: If it ever happends that Lukas gets damage, then he atleast got passive healing going for him every other turn.

Threaten Atk 3: This makes Lukas outnumber the Atk of his foe easier with his Def

Close Def 3: Gives +6 to not just only physical Atk's, but also gives Lukas a bit of bigger chance to survive against dragons.

Ignis: To show you how good this works with this build let me give you some numbers.

If Lukas Attacks: Def = 45
If Lukas IS attacked: Def = 62

This makes it so that ignis does:
36 unblockable dmg when he attacks
and
49,6 or 50 dmg when he is getting attacked.

And reminder, his ult is charged in two attacks if the foe initiates combat.

Weakpoints:

Obviously his lack of Res which makes him an easy target for dragons, Tomes and priests.

And his lack of speed makes him easy to be double hit, which can end up being really bad if a magical attacker is attacking.

It';s fairly self-explanatory. I just wanted to make a Lukas with stupid amounts of defense (that is semi-affordable). Also, you will get crazier stats with all ten merges and dragonflowers.

Obviously, you have to start with Def boon. You could go with either a Spd or Res bane, but I think Res is the least salvageable.

The Daybreak Lance is probably a more logical option, as it is already in his kit and boosts both Atk and Def, but again, I'm just having fun with his Def. So, Vanguard+ with Def it is. (plus I had a 5 star Thea lying around)

Reposition because, eh, why not? You can use other skills like Draw Back or Pivot, but not skills like Smite or Shove, you'll see why soon.

Bonfire or Ignis because, whoop-dee-doo, defense! I chose Bonfire just because it procs quicker.

Steady Stance 3 because, say it with me now... defense!!! You can use Steady Stance 4 if it is available to you, but 3 is way more affordable.

Atk/Def Link 3 provides Lukas and another unit with Def! ...and Atk, but mainly Def!! Honestly, I find Quick Reposte 3 to be more useful here, but I am taking Lukas's Def to the max, so Atk/Def Link it is!

Odd or Even Def Wave works, I just had Odd immediately available at the time. Again, Def!!!

I chose to go with double Steady Stance 3 because... well, why not have a little craziness in your life? Really, you can do a lot with the seal slot: Def 3 for pure defense, Atk/Def Bond couples well with most position-based support skills (sorry Smite and Shove), Armored Blow works so you have extra defense for both phases of combat, Even/Odd Wave Def skills for more doubling up, Close Def 3 it you want to patch up his Res a little, the options are endless!

Well, there you go, a Lukas with crazy Def. Worst case scenario, he will have his base 41 Def, and in best case scenario, he will have the insanity of 76 Def! And that is without merges and dragonflowers, which would make the number even more stupid!

And lastly, if you were to apply these skills to Picnic Lukas, who has even more base Def, you could get a max of 81 Def!!! Again, without merges or dragonflowers!

Have fun with a Lukas that is extremely good at his job, but can't do much else.

While at first Glace Lukas may not seem like the type to run Distant Counter but with this build Lukas can reach a solid 38/37 def/res not to mention any buffs, If invested into enough, say multiple merges, you may find the Distant Counter Build Quite Useful for a +10 or even lower Lukas.

While Lukas's Refine is quite amazing, for this build to help him with his Res we use Berkut's Lance (+Res), or Barrier Lance (+Res)

His movement skill is up to the player

Since he is lacking the Slaying Lance Effect from his default Weapon we give him Bonfire instead of Ignis, being able to pop a Bonfire in a single enemy phase encounter, other 2-3 cool-down specials work just as well and even Ignis can work out.

For his A slot the previously mentioned Distant Counter is a good use of it for this build, alternatively to DC a good use would be Fort. Def/Res bringing him up to 44/41 Def/Res but lacking the Distant Counter effect.

Lukas uses Quick Riposte to double seeing his quite lacking speed, but if the player thibks the double isn't necessary Guard can always be useful.

Lukas's C slot is quite open to player preference, buffs such as Joint Hone Def or Odd Def Wave help him stay alive or even a good panic ploy seeing his grand HP.

Lukas's S slot is in this build used to help bring his res up to take on DC and Dragons, Seals such as Warding Stance and Distant Defense help with that goal, alternatively to pair with a Joint Hone skill, he can run Def/Res Bond on the S slot.

This Build is all about tanking any and all melee units of just about any color while the build is very expensive i believe it is worth it he can be the most reliable unit in your barracks if you build him like this just... keep him away from mages whatever you do it will not be fun if you don't