He has five main weaknesses: A. His confidence would get him in trouble; he just doesn't know when to stop talking and he might underestimate his opponent. For example, had that bandit chief been stronger/larger etc. he could have possibly hurt Ontus while he was monologuing at him. B. Additionally, there isn't altogether too much he can do against magic unless he has preparation (Resist magic potions), but that's unlikely as most fights aren't planned in advance. C. If they get inside the reach of his spear he's in serious trouble as a spear is mostly useless once you are within its reach. Furthermore, a skilled mage or bladesman could time an attack during a parry etc. and cut, damage or destroy his spear, thereby forcing him to use his sword (Which he might not even have and is considerably less skilled with) or a less effective half-broken spear-end. D. He's lightly armoured, so though he is quick and skilled, he is more open to wounds. E. A spear can be difficult when your opponent is in full, thick heavy armour (Say, ebony or daedric, dragonbone) as even with a sword they can be hard to penetrate, even more so with a spear. He'd have to hope he can get some shots on the armpits or achilles tendons (Both usually chinks in armour sets). Hope this helped
Haha
Fenrir fighting Ontus would be interesting, but I think if Fenrir could get within spear's reach Ontus would go down, though he'd probably try and make sure he at least cuts Fenrir in order to achieve posthumous victory haha, though that would be difficult. If Ontus could keep him at a distance I'd say Ontus: shirtless against a spear-fighter isn't a good idea, especially if said spear is poisoned
but it would be rather interesting