You might be spending a bit too much mana. Try to get the monsters as low HP as you can without having either one kill the other, so you can use a lower-level DoT to finish them off (DoTs also preferable to nukes since they do more total damage for less mana). Not really any need to snare or root when you're finishing them, either; they should already be fleeing, and slowly because of low HP. Manage aggro between your pet and the other monster so that both are very close to death at the same time; you can use cheap spells like snare and flame lick for this purpose.
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