Stop the Behemoth is a mission in The Lost Frontier. After fighting the Aeropan siege on Far Drop, Keira alerts Jak to the ACS Behemoth approaching Far Drop, telling him they need him in the air to mobilize their defense.
Once you exit the pirate town you will quickly be greeted by the Behemoth, fully repaired and with a powerful new weapon. It is accompanied by pairs of fighters led by heavy fighters. Shoot these down as much as possible to lessen the strain on your plane, as you will be largely occupied with the Behemoth and its array of anti-air turrets, which you also will want to target. Despite the large amount of possible targets, your first objective is the crystal on top of the Behemoth, which must be destroyed before the timer runs out.
After the crystal two flights of four fighters each will attack you, shoot both of them down, followed by chasing after the Behemoth to destroy a second generator crystal. Several heavy fighters will now take to the sky, presenting your next target. These are quite sturdy in comparison to the fighters, but are also slower and make an easier target. The Behemoth will use a third crystal to try and fire the weapon, which will be rendered disabled once it has been blown up, marking the end of the mission and another retreat from Skyheed and the Aeropans.