Torn between Black Mage and Summoner!

[Havenchild]

You generally don't need mana in any fight except T8, in most cases you won't ever need to use Energy Drain unless you die. As for Ruin II's, you should only be using the skill during movement, inbetween off-GCD's (such as Rouse, Spur, Kindle etc) or right before a fester in a Ruin I rotation.

Ex Ruin I -> Ruin I -> (Festers about to come up -> Ruin II -> Fester

Using Garuda also lowers your MP cost over due to contagion.

Don't use Miasma II unless you couple it with a Contagion(for single targets). Try not to clip your DoTs too early. (Recast at about 3-4 seconds for most fights). Make your best out of Bane in multiple enemy scenarios to prevent having to recast fresh DoTs on extra mobs.