Threaten ATK would work as a skill; but you may also want to consider ATK Smoke or (if you can go more premium) Threaten ATK/DEF instead for the self-buff as well. And you could use Distant Foil (again, if going premium) in the A-slot to stack more ATK and DEF onto him.
IMPORTANT NOTE: You should definitely know what you’re trying to build the unit for. An Aether Raid build will be different from a PvE build.
But yeah, generally speaking, @Tandor was right. Making a build that syncs with their skills is more or less the best way to go. What you could also do is to plan out a build to cover a unit’s weaknesses. Two examples I can bring are my Say’ri and my Edelgard.
Both builds recreated in builder because I’m not on my phone.
This is one of the many builds I use on my Say’ri. Normally, she’s a fast mix-phase unit with Amatsu and Close Call, but I drafted a build to make her into more of a dedicated counterattacker (even though I generally only use her with Slaying Edge and Galeforce, she still has this option available.
To make up for her lack of physical defense, I started by giving her a weapon that would cover her weakness, Safeguard, and refined it for that stat as well, giving her 40 DEF/RES on defense. Because she sacrifices Amatsu’s DC, I put that in her A-slot so she can counterattack anything at all ranges. As a counterattacker build, Null-C guarantees that she can always counter, and the Oath and Stance C and S skill ensure that she is difficult to double and hits hard on the followup. Alternatively, I can also give her Close Call instead of Null-C so she can tank even more, but that’s more for PvE.
Edelgard’s weapon plays more to her being a loner bruiser unit, relying on being outnumbered to make up for her lower SPD. So what I did instead was I gave her a powerful, speedy, one-hit nuke build that I based off my Galeforce Quadding build for Sieges. With this, she’s harder to double while still being able to pull off a powerful AoE bomb every initiation. I built this around the Blazing Wind Special, so I gave her an A-skill to maximize its damage, LD4, and a B-skill to help ensure that it’s always active (Spiral). But she still needed a Slaying weapon to ensure its activation. I could stick with her Victorious Axe, but her higher SPD makes its effect less useful, so I gave her a Slaying Axe instead so I could refine it for SPD or ATK, whichever would be needed. The Rouse adds to her still-high DEF while also pumping her ATK up so a solid 80 points for her Blazing Wind, and Hardy Bearing ensures that she can’t be Vantaged.
Both of these builds are made in a way that makes up for a general weakness of thiers, Say’ri’s DEF and Edelgard’s SPD, and both are highly effective as well despite not 100% capitalizing on their native strengths. So this is another route you could go with it.
(Also why some people make speedy builds for non-speedy units.)