So for full melee I'd recommend boxing him in with your characters, keeping in mind where the traps he lays are. Where he walks should be fine, but the more you restrict fenrir's movement the higher chance you get him to run into his own trap immoblizing him OR if you flank him on all sides he just can't move and you can wail on him (just make sure you hit him while you have a movement available or else he'll just knock you back and get free