Solo mages are not a problem. Aside from the questionable stun/ice rate, but let's not get into that.
People seem to have a problem with coordinated mage teams. In all fairness, a team of any class, coordinated and operating on TS or Discord, can do equally as much damage. I've been in coordinated mage parties most of my time in KO and I can guarantee a coordinated melee team would wipe out mages any day of the week.
People play with random parties and expect to perform on the same level as parties operating on TS and Discord. Odds are, these coordinated teams have probably also been playing together for years, and so know each other's playing styles, inside and out.
For mages to hit this kind of damage they need to be full mp which means seriously low hp, to the point an assassin could one hit with CP and a warrior could 2 hit.
From what I see people seem to run straight at mages with no real tactics hoping for the best. Ultimately, try harder. For example, warriors have descent, use your assassins to get close and descent on to them. The amount of warriors I see that don't seem to even have descent on their skill bar...it's crazy.