I'm trying to take lvl 21 pirates, my cav is purp lvl 52, full legendary 41+1 carrier gear. Working..

You can consistantly kill 21 pirate with a 52 purple cav with overall attack and armour at level 10

Main consideration are the planes you use. To consistently attack lvl 21 pirates, you need 5 star gold planes to match to the carrier, assuming you are attacking pirates without warships. Also, damage done by pirates to your carrier must be less than 1.2 million each battle to ensure that the carrier can survive for the next fight.