magic has its own skill bar, so, it should get better over time like other weapons. though, i dont know if the skill bar actually affects damage, or just mana usage.

Magic is so far weaker than bows or melee when you look at sustained damage, I play it more as off weapon, bursting my Eitr out and then switching to melee/bow, rotating back whein Eitr is full. They will likely adjust balancing, frost staff needs a physical element and some of the fire damage should get moved from the DoT to direct damage, as the DoT doesn't stack.