Halo Wars: Definitive Edition

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.

By far the easiest and fastest way is to use wolverines. Train max units and go up to him and volley. Weave your way through the left side of field to miss all nodes. Same as the 360 version

This can be a somewhat difficult mission specific achievement. The strategy I used was to have Gauss Warthogs weaving through the Scarabs laser in the opposite direction of its movement, so the hogs could pass it safely, and used mac blasts whenever possible on the middle of its body.

WARNING: The Gauss hogs may attack nodes as you drive past them, so be careful to not destroy them on the way to the scarab.