I don't like using dots to finish mobs off. It can break the root with the upfront damage and then you'll be using more mana to root the mob again while the dot kills. Might as well just pick the right-sized nuke to finish the job without too much overkill.
I would always make use of the variety of slow spells we get to try to keep mobs even-ish on HP even if they aren't the same level. Between languid pace and tepid deeds, cast somewhere between the start of the fight and halfway through the fight, you can get any mobs within ~5 levels of each other down to <10% at the same time to finish them off easily. Knowing the levels of different names of mobs, or watching for max hits to see what level they are, is very helpful for this kind of thing, of course. The sooner you know what spell you need to use the less mana you will need to spend keeping mobs even and finishing them off for full xp without slowing down your overall pace... although if you get a really tough mob it isn't a bad method to use two weaker ones on him and get full xp for the three without messing around with slows, that just doesn't happen all that often really.
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