What's the best build for Luka? Not good in doing that so I have no idea how to bring out shota's max potential
Basically, ignore everything other than attack, and put the accessory that sets his HP to 20% at the start of a fight and gives him +50% to all stats (technically you can set his health low without that accessory with the orb of lifedrain, but it's pretty tedious needing to do that so I recommend using that accessory). Give him the crisis extra action skill, the skill that gives him +100% to all stats when below 20% health, and a bunch of other stuff that increases damage, plus one of the haste skills that lets him attack first (either heroism haste or whatever attack chains into heroism haste). Use gems that buff heroism damage and attack in all of the slots of his equipment (plus the crisis 3x action gem you can get from the reaper battle challenge, which brings him up to 4 actions), and pick every ability you can find that buffs his damage, and use your strongest heroism attack.
I'm not sure if it exists or not since I haven't unlocked everything yet, but if there is one, then special haste would also be really good because it would let him use the special skills that give huge stat boosts for a turn (when you have 4 actions per turn there's no reason you wouldn't use them).
Also, if you can't kill an enemy in 1 turn, then just have your tankiest character use the medicine skill that resurrects someone with 20% health and you can do it every turn (as long as Luka doesn't have HP that's a multiple of 5 you'll keep the crisis buffs with that medicine skill).
.. I should note however, that you could use this build on basically any character and you'd probably be able to one shot the final boss with basically anyone doing this if you had strong enough equipment and unlocked enough jobs. Luka's ability is nice and all and it does make it a little bit stronger, but.. honestly, tripling the damage output of heroism skills isn't even that big of a buff in the grand scheme of things when you consider that you're multiplying his damage by something like 800 from just having the right gems, maybe even more than 800 if it's fully optimized.