Special Fighter on Arden would be used mostly for its guard effect; like for PvE maps where he needs to create a chokepoint. He doesn't use brave sword; I got pitybroken by Lene thrice and one of them went to Arden for safeguard+.Another 17 orbs dumped got me another L!Eliwood (SPD+/RES-), though will just merged him into my first one to negate the HP bane.
And not certain about recommending SF on Arden. I mean sure he has a brave sword, but if he didn’t have such slow speed he would have given off four hits. Usually that skill is recommended for armor units with naturally high speed or units that have skills that guarantee follow-ups. The latter case being B!Ephraim with his Garm.
Though I doesn’t hurt to do a bit of experimenting.
If I ever do invest in Arden as a supertank (big if, given his terrible resistance), special fighter is useful with QR3 or one-shotting in AR for special acceleration on foe's attacks. He can get instant noontimes, etc. The skill set there would probably be barrier blade+ (+res)/DC/Special Fighter/Attack Smoke/(QR3 or Fierce Stance).