I'd like to add a couple things.
1) get a gaming keyboard and learn how to use it with keybinds, this done efficiently will boost your gameplay 7 fold.
2) if you're a dps, always carry health potions, they can make or break a match, but dont waste them when you're against a premade or coordinated team unneeded.
3) already somewhat mentioned, stay grouped up, always, going in 2v10 will only get you killed, pay attention to your teammates position on the map.
4) dps, if possible, try to keep whetstones / spellcrystals on your weapons, this can get expensive, but if you farm for them, they will significantly increase your output.
5) have a good planar focus setup, a good planar focus with good essences plus good item enhancements on every slot are highly recommended.
6) know when to kite, when you notice people focusing you, pop cooldowns, run back to your team, this could save you a GY trip.
7) learn the other callings. Know their cooldowns, when to swap targets.
Some of these may have been mentioned but still need strong reiteration.