Use Protect and Bubble to up your survival rate, then get in there and beat Fenrir down. There’s not much to this fight, but it does tend to get bloody. Fenrir can cast Bravery on himself in a pinch, or use Wail to inflict Blind or Silence on all characters around him.
Hashmal is a fast physical attacker the way Fenrir was, but his standard attack often hits more than once and inflicts Disease. Make sure you have plenty of Vaccines with you. When he gets below 50% health, he’ll start throwing Earth-based spells like Quakeja or Roxxor (…ugh), which you can safely ignore if you’ve cast Float in recent memory.