-Atk +Speed Hardin

Well, at the very least it helps for surviving.
What you can do is find a way to buff his speed. With a buff he doesn't need speed skills.

Don't invest any skills in his Spd. Maybe the Darting Stance seal would help him use it on enemy phase if you find that he is dying due to being doubled. With 37 Spd from the seal he already avoids a lot of doubles. If he has a Hone Armor active, that's 43 and only the fasted units can double.

But in general, Hardin uses his Def, Res, and HP to survive combat, not his Spd. The best Hardin build is Warding Breath, Bold Fighter, Quick Riposte seal.