When to use slow

[Sadana]

I cast Lethargy on the big pudding boss in Wanderer's Palace whenever he targets someone.

[NyneAlexander]

Heavy: dreadknights in T5

Slow: the mage add in leviathan at 80% health.

Since both my heavy and slow come from lethergy, which is off of the GCD, I throw it on for those two adds and others like it.