expiration

If stat increase is all what you need then go for it.
Personally, i would keep his breath since it's a bit expensive for +4 to res stat and 5 to hp stat boost. His a slot skill is open so you can use fortress res for that reason (even as a sacred seal) or consider waterboost since his hp is so high, especially for ranged threats. But in the end, the choice is yours.

Well, it's at first view a waste of refining stones (as other units could use them). Second of all the lack of said cooldown and the higher might is what makes him a monster, as it simply gives him one of the highest atks (if not the highest I believe) in the game under the right circumstances. This is only compounded upon when he procs Ignis or simply hits you twice with your choice of Bold/Vengeful fighter.

My arguement against the refine is as follows. Do you truly need it? It's hella expensive (20k feathers, 50 refining stones, plus the unit you will sacrifice which could be used for merges such as A!Tiki or Nowi) and it gives you no benefits to arena scoring if I'm not mistaken (as prf weapons count similarly to refined weapons in value). That, and 4 res wont do him alot of good, as he's already beyond slow, and any red or green mage with high atk and speed (the majority of green mages) they will blast him to smithereens, and even blue mages will leave a dent on him. Add unto that, that the loss in cooldown reduction and atk will remove some of his prior kills, and not only hinder his special activation (which if Ignis, procs every second hit, and Aether which procs during his second fight)but make them activate much later (Ignis proccing during the second fight and Aether proccing during that same second fight). I for one vote to note eliminate it, as it only adds 9 bulk to ranged (4res 5 hp) and truly does him no good, as it may even limit the kills he may or may not get.

Well, he's damn fun if you can give him Aether and Steady Breath with his Vengeful Fighter because Expiration has distant counter & preferential targeting built in with no need to refine and he'll have lots of long term sustain because SB and VG provide extra charges when attacking and getting hit.

And as the other comment said giving him Distant Defense seal would be fantastic for him. Mine only has 27 res, but does really well without any extra res boosts, though I do run 4 Ward Armors in my Armor Emblem teams too, for a total boost of 12 res for him. ^w^

Finally, think of it this way, taking away 4 atk is really losing out on 8 damage because he attacks twice with VF (4 per attack)

Ultimately, it also depends on what you're asking him to do too. I don't think you can pump his res up enough to take on powerful red tome, Naga tome, or Falchion units; those spell his doom, even with my extra wards. Unless maybe you have a +10 merge . . . owo