When to cast Slows

[eqravenprince]

I have my enchanter up to level 29. When should I cast Tepid Deeds? On easy mobs they die so quickly that it seems like a waste of mana. On hard mobs, the mob instantly aggros on me and does not go back to the tank until mob is under 50% health. And even cons or higher, I don't even bother casting cause it will get resisted 80% of the time and that is even with it tashed.