At the moment I'm writing this, there are 6 classes total: Fire, Toxic, Freeze, Wind, Eletric and Ground.
You get a talent on Level 3 and 6 of each class and the starting ones doesn't count towards this cheevo, so you need to reach level 6 with each class ir order to get this one. Level 6 is relatively fast and you can get to level 6 with one class a day by casually playing.
The experience is shared between your mage and your classes, wich means you only get one type of experience at the end of a match. Altough the xp requirements are not the same for a same mage and class level.
The most efficient way to get xp is by time survived and damage caused. Accolades give you so little xp you shouldn't bother going for them, except the ones for finishing 3rd, 2nd or preferably 1st.
Important: You can only level up your starting class, wich means if you start with fire, doesn't matter your other gauntlet during the match, you will only get xp for Fire.
For those who haven't started the game, Talents work like perks wich gives you some kind of in game boost (mana cost for using spells, damage caused increase, damage receive reduction and many others). There are 3 types of talents: Mind, Body and Spirit and you can equip each of one respecting the 6 points max combination, wich can vary from 1 to 3 for each talent. Also talents can be upgraded in game to increase their bonuses by reading certain scrolls.