Spike is an intermediate level mage, or more accurately an anti-mage, bolstered by his own natural affinity for magic and his resistance to it. His offensive spells are fire based, being a dragon and all, but have magic nullifying effects which cause them to not be as powerful. Defensively, his buckle provides additional defense against magic spells. He has moderate physical ability and strength, but to compensate for his lack of a physical defense, he also specializes in teleportation and portal magic. The spell book and pendant combo what allows him to channel portals, while the bracer allows him to blink short distances. His strategy in combat is to counter other mages, and keep his distance, as hes provides the means for escape.
He seems to see himself as that as well.
I like the idea of him being the physical contrast to Twilight’s magical speciality. They’d make the perfect team that way. With the added bonus that his dragon scales make for a nearly impenetrable armor, and his dragon nature makes him extremely resistant to magic, he’s be the perfect tank to support Twilight’s extremely “potent, but squishy” archmage style.