Repel the Aeropan boarders is a mission in The Lost Frontier. After finding the third coordinate sphere and being ambushed by the ACS Behemoth, followed by forcing it to temporarily retreat, Jak returns to the Phantom Blade. When on board, he receives a call from Keira to move to the upper deck. Once at the deck, Phoenix relayed to Jak that the Behemoth has grappled onto the Blade and orders him to help defend the ship.
Once at the deck you must defeat the Aeropan commandos that have already boarded the galleon. Since you are prone to running out of ammunition and health packs due to their large numbers it is recommended to use any of the rapid-fire turrets on the deck to defend the ship. They are not destructible but any damage dealt to the turret is automatically dealt to you, the enemies are oftentimes more focused on attacking the ship than you however. After the first few boarders you will have to destroy an Aeropan gunship, which launches rockets at the galleon. Shoot both the rockets and the gunship down with a turret on the portside of the ship.
Now the turrets on the Behemoth will begin opening fire, use another turret on the starboard side and destroy the enemy turrets, as well as the armor behind them to damage the ship itself. More boarders will arrive now, followed by two gunships once they have been defeated. And after the gunships, the Behemoth's turrets will open fire again, though this time you can use the Blade's lob turrets, accessible by using the doors with a green light on either side of each lob turret (both at the front end of the deck and near the rear).
The lob turrets have the added advantage of massive area of effect damage, but cannot hit anything on the Blade itself, in turn, you can destroy the Aeropan commandos while they are exiting their own ship, shortly before they board yours. At this point the mission will start repeating itself, but with the added factor of not limiting itself to one specific method of attack anymore. Now both boarders, gunships and turrets will begin to attack you in both greater numbers and all at once. Begin with focusing on the commandos, ideally by using the powerful Lobber weapon, followed by the gunships, followed by destroying the turrets. The turrets are relatively weak, but ignoring the boarders and gunships will likely cause the mission to fail before the Behemoth has been damaged enough to force it to retreat.