A very safe way is to only use MAC blasts to destroy the scarab. Play on easy to maximize damage of the MAC blast (press up on de d-pad twice). It takes a bit longer than using scorpions/wolverines but it is without any risks of accidently destroying a power node, which are quite weak. Build some turrets and ground units to your liking to defend your base.
Edit: I just saw that the 360 guide from I TeMpLaR I UK suggests the same. He also advises to use the emperor skull to recharge your MAC ability in half the time in would normally take.