There are three ways to defeat a Scarab. 1.) Target one of its front legs with a somewhat powerful weapon. Missiles, explosives, any vehicles fire, etc... and as the armor falls off, it will eventually drop to the ground and sit still. At that moment, you need to climb onto it from the side ledges, the side doors, the open backside, any way you can. Once you're on it, get to the main deck outside it and walk to its rear. You should see a large glowing wall. Shooting it with anything a few times and the whole Scarab will blow up in 20 seconds. 2.) You don't need to damage a leg to make it accessible from the ground. If there's a jump nearby or somewhere up high that you can jump from and land onto the Scarab's back, then you can just walk to the back and destroy the power core like in the above explanation. 3.) You usually can't do this, but it is possible to shooting what can only be called its large glowing butt to destroy the wall blocking your line of fire directly into the core from far away. With any powerful weapon or vehicle, a couple shots to this part will cause it to explode. You can then try to shoot the power core of the Scarab from far away with a vehicle, rocket, or sniper rifle.