Following info comes from Mega 16.
Here's Behemoth.
Here's a shot of Red Ranger holding onto Behemoth while the girls are armed with their "Astro Blasters" getting ready to fire.
Here's another shot of Behemoth.
Behemoth was defeated by the Spiral Saber Booster Mode and the Quadroblaster.
Here's Giant Behemoth.
Here's a shot of the Astro Megazord Blaster's Beam hitting Behemoth which destroys him.