Whay does delay skill do exactly? does it just debuff target speed? Or does the target loses its turn entirely?
If a Monster has a Delay applied to it, it will take its turn after any Monster without a Delay skill. If you have more than one Monster with Delay applied, they will take their turns in Speed order after the Monsters who do not have Delay. It also removes any First Strike effects on the Monster when applied.
You can find out more about Monster skills by clicking the question mark next to the the word Skills on the Monster Details screen in game.
I hope that helps!