Every spell level increases your power pretty much.
Canni allows you to create mana. Regen helps you canni more. You need to learn the balance. Canni enough, but not too much. Usually I would canni so my mana was higher than my health and stop. Then canni more as my health went up and my mana went down.
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Poison dots are much faster than disease dots.
Grouping (or duo/trio) really is the best way. Soloing will get better though at 24.