Leveling up isn't exactly the hardest thing to do in-game.
But here is one usefull tip...
CHOOSE A SIDE!!!! WE'RE A WAR!!!!
Oh yes, the Warrior Vs. Mage war.
Be a mage, because they get the highest attack damage rates anyway.
Anywho, just get really good at doing one type of attack, then keep clicking Battle Mosters, or go on your own little quests. Keep changing stuff up so you don't get bored though.
Hope this helps!