If you don't have any spammable attack other than Agony, don't spam it! Main offenders of this are Crawling Claw and Creepy Crate.
Agony will do more damage on subsequent ticks than on tick no 1. If you reapply it each turn you will only do minimum amount of damage from tick no 1 over and over. It is actually more effective to just pass instead of casting it again on the same opponent.