Time to put in my two cents for the newbies.

-The mage should always put mana, healing and that magic stick thing(DON'T SAY IT!) above everything else, as they are the most useful things for a mage.
-Beginners should not use mage, as you need a lot of skill points to be effective with one.
-When the castle is being overrun, heal the CASTLE, not players. If the players die they can be revived in the morning. But if the castle falls it's game over.
-The mage(who should be at least lvl 10) should go on one side while the warrior and ranger go on the other. The mage can easily defend itself, but the warrior and ranger by themselves are usually useless.
-Rangers should put points into Frenzy, attack speed and especially mana. Having Frenzy fully leveled up is extremely useful with high attack speed, and having high mana allows for using it more than once.
-If one person quits, the other two may as well do the same. It's basically impossible to get to a high wave with only two players.