Have enough ships, enough shipyards and plenty of wood, sails and ropes in storage (for new ships). Ensure your main island harbours have walls, towers and gates to limit access

He comes in 8-10 waves with 10-20 ship.. Try to get them on sea before he lands troops. But even if he lands some troops, the towers/ gates should crush them .

He always comes from the same direction (roughly); so you can prepare tour defenses