Since Noxious Fox’s attacks lower its targets’ defense, it’s a good unit to use when swarming large, tanky units like Faceless Saint or Jacob, especially because Beasts lack any hard removal.
When NF gets below half health he’ll get Enraged, increasing the speed at which he debuffs the enemy, making him incredibly useful. It might be a good idea to try using Blood Ritual to get it Enraged, then using a healing spell, such as Bless or Miracle, to recover that lost health. (Note: Enrage doesn’t go away once it’s triggered, even if the unit goes back above half health.)
Noxious Fox is a bit like Venomite, but while Venomite does AoE damage, NF just hits a single target. Since it’s only single-target, Noxious Fox is quite vulnerable to being surrounded by multiple units, such as Brutaxes or Wyrmlings.
Another thing you can do is use units that will lower NF’s attack speed, such as Poison Gecko or Spike Boar, so NF can’t get off as many of his debuffs. Finally, you can stun it or freeze it with certain spells or units, such as Cryonia, RIPhorn or Snowy Owl Fighter.