If you're a member, go to Nightmare, the room with 4 level 99 Nothings (yes, that's what they're called).

If you're a free player, go to Ice Storm Arena, the room with 5 level 99 Frost Spirits or whatever they are.

For the most part, especially if you are buffed and most especially if you grind class points on a double class points day, it takes less than an hour (sometimes only 15 or 20 minutes!) to grind up a class.  The hard part is obtaining the class itself usually.