how should i build?

Well you can go for:

Defensive build:Give him fury/Close defense/steady stance/breath and +def on his seal(Fury and +def seal are budget and saber has good physical bulk)

Or

Offensive:Give him LnD/Fury/Deathblow,Vantage/Desperation,and +atk/spd seal.(use Fury if you want a budget build)

You came to the right place.

With Saber, his IVs are so universal that he can run a number of different builds. I personally have a +def-hp Saber, and with his sword refined and fury, his only stat below 38 is his resistance.

But in your case, you’re going to want an offensive bruiser since he has a very respectable 34 base attack to go with his 33 base speed.

I will list a budget build, and a not so budget build.

Budget build:

Refined slaying edge for speed, reposition or swap, Ignis, fury 3, desperation, C skill of your choosing, and a speed +3 seal.

With that build, Saber’s offensive power hits 51 attack to go with 42 speed, while still maintaining 35 defense. Very respectable across the board.

The not so budget, offensive build:

Slaying edge refined for speed (or attack if you want), reposition/dual rally, Ignis or Aether (probably Ignis), life and death 3, wrath 3, C skill of your choice (probably Drive Spd 2), attack +3 seal or heavy blade seal

With this highly unorthodox build, you reach 53 attack (55 with the attack refinement, 56 with just the seal, or 58 with both) and a minimum 38 speed (41 with the refinement). Heavy blade can work pretty well if you run an attack refinement and have him early in combat, but it may be better to just power him up more to a 56/41 spread. One round of combat should put him in wrath range, and you can let him charge up before going in and dropping an extra 34 damage on your opponent through the +10 extra damage, dealing up to 90 potential attack power in a single swing before buffs.

With a very good base attack and a workable base speed stat, those should be your main focuses. Everything else does not matter as much, because he will still have decent enough bulk to get by, and his res is bad enough with the bane to where it does not matter too much if it goes lower.

The choice is ultimately yours, but I hope those ideas help.

I mean, Saber just isn't that useful right now. There's just so much better of Sword units out there, like Zelgius, Ayra, Siegbert, Mia, VL Ike, OG Ike, Alm, Lucina, Fallen Celica, and like 20x more units. If you really wanted to build him, I'd say try for something along the lines of:

Slaying Edge+ (Defense refine)
Reposition
Bonfire
Fury 3
Vantage 3
Threaten Atk 3
Quick Riposte 3 Seal / Close Defense 3 Seal

Or something along those lines, perhaps. That'd be a physical tank build. As someone else said, Saber is pretty flexible set-wise.