Equip carronades on your ship, chain shot enemy sails down to below 70%, sit on the stern of the enemy ship and fire into it, when the stern amor is gone switch to grape and fire into the stern one shot at a time using spacebar until they have minimal are or half of what you have, switch to boarding prep while doing this, when you have lowered their are enough and boarding prep is at 100 board the enemy ship and you should have an easy time.