It's kind of hard to determine the classes from my perspective. I was going to bring up the User's Skill VS Hero thing myself but I'll avoid that. As to my concern, it's really more or less how predictable some classes are to others.
By that, I'm pointing to the majority users of each class, not your exceptional super professionals who know what to do in every situation
For example, most fire mage you encounter attempt to use the two hit combo to knock down then meteor, or meteor>teleport>flame wave. Majority of Berserkers you see tend to use the war horn at whatever time they have it, then position themselves to use the hat to ram them off the edge.
So really it comes down to how predictable and how easily you can find methods to over come those heroes. Honestly I find the Heroes in Lost Saga the most balanced compared to other games I've played.
Nothing seems so overpowered to the point that it's really noticeable
I only find Medics annoying (for drain reasons) as well as ice mages. Overpowered? Far from it, I find it all balanced out the way it should.