I finally get Leif!!!, but...

In my opinion, people should only worry about the meta when it comes to units they actually use as their main arena team, which is only 3 at the bare minimum. The 4th slot is usually reserved for the bonus unit, which many other easily merged units could achieve. 35 spd is till pretty solid in PvE content and with his skill set, he's a master Tempest Trial or Chain Challenge unit. With cavalry buffs he'll be good to go. Steady blow is already useful since it grants him more spd on inititation and makes him more bulky as a duelist. Drive atk is a nice skill to have in mixed teams but otherwise cavalry buff when riding with other cavs. Give him Moonbow since he has middling atk and you're set.

If you really want to use him for arena, especially since he'll be a bonus unit, then quick riposte B skill will help him ensure doubles because he's tanky enough to take physical hits. He can do well vs dragons as they usually have less speed than his base 35. He should say clear of mages though unless he gets the jump on them and ko them or he's sitting duck in the enemy phase vs magic.

Fury 3 is a good bet. Great for PvE with multiple rounds, like TT and CC. If you wanted a wacky build you could do Life and Death 3+Hit and Run.