My character is a massive glass cannon, he has an incredibly limited amount of time to unleash a lot of devastation, and can be killed easily in physical combat, almost as easily as a regular human. With that in mind and all the sacrifices my character's made in other stats in order to maximise his damage potential, he'd still be completely unable to damage that character, while also being killed in a single punch.

My advice, remove the majority of your magic resistance, having passive near immunity to a wide range of magical effects is BS at this tier level. Reduce the damage of his hits, or reduce your movement speed, because at the moment your character is fast while also packing an insane punch while also being near impervious to harm.

Remove the BS 'weakspot' shit that no one can exploit without meta-gaming. Have your character have a highly finite amount of resources, because regenerating isn't supposed to be that efficient at this tier level. He could still be relatively impervious to physical harm such as small arms, physical blunt damage, etc, but he must still be harmed by prepped physical attacks and explosives.

That's how I would balance it anyway, ultimately it's up to Skallagrim how you balance a character that's not really weak in any particular physical stat while also being highly durable and resistant to magic of pretty much every kind.