Warthogs are you biggest ally in that mission. Never send a lot of units at the same time, small squads with enough firepower is enough, locusts are your biggest problem so deal with those first. After dealing with the power batteries of the scarab keep sending squads to take it down and keep the warthogs moving even if the laser detects them, use a small portion of those warthogs to distract the scarab while keeping the other portion alive shooting at it.