Lukas Merge

+ Def Weapon Refined = 54 Def
+ Summoner Support = 56 Def.
With a +10 merge *-*
That would simply be awesome.

I would recommend one of two builds, the budget build and the really expensive build.

Budget build;
Slaying Lance+ (defense forged)
Swap or a reposition of your choice
Ignis or Bonfire
Defiant Defense
Seal Attack
Threaten Attack or Threaten Defense
Close Defense or Defense +3

Basic build idea is to make him a 1v1 monster. Once he is under half health and he has proc'd Seal Attack it gives him an effective 14 extra defense. Oh, and I legit 1v1'd an arena Hector with this build. Even if a unit can get you to half health, they usually can't get you much lower. You only have to be careful around super heavy burst axes, really. (Cherche's a bit scary, so is a forged Minerva.) This was my original build, the fodder is cheap and easy to come by, and it is very effective.

The more expensive build is as follows;
Slaying Lance+ (defense forged)
Swap or a reposition of your choice
Ignis or Bonfire
Steady Breath
Guard 3, Quick Riposte or Cancel Affinity
Attack Smoke
Close Defense or Defense +3

This is the one I have now. Waaaaaaay more expensive, but it turns him into a far more dangerous unit, at the expense of a slight amount of survivability. (A veeeery slight amount.) I'm going with Guard 3, since I have the fodder, but QR3 gives him a KO when being attacked (and having Bonfire, I like Ignis' larger numbers) and Cancel Affinity gives him more matchups against Emerald Axes, though that one's more situational.

Also, hone for defense, there's no saving his RES, you want as low of speed as possible to charge your special, and he hits harder with his special than he ever will with +2 to attack.

I also had been working on a Lukas ignore the set (since I had different options) and what I can tell you is make him a character that stands out on enemy phase ignore player phase go all out with DEF mine is +atk and works exceptionally well with his normal defense I think fortress def is not very good option, he doesnt need it that much I prefer something like atk/def bond or close def even def+3 works well QR is a must with ignis. and threaten atk. try to use reposition instead of shove or swap since he can be in placed of someone to receive atk. and if they ever do a quick ripost seal use something like lancebreaker or sword breaker since his atk isnt that bad.