There are 2 ways to get this done.

1.

Go on deathmatch mode (I always choose the prophet) and go on a map that has a empty base location. As soon as it starts build a temple, a shield generator, and turrents to protect your base (and anything else you want to build). Then send your ghost to the empty base location and as soon as that base is built start building a scarab. Have your scarab and leader attack your opponent.

2.

Go on any regular skirmish match on any map that has 2 free reactors on it (choose to be the prophet for best results). When you start out build a temple and 3 warehouses, send your ghost to get supplies (and your leader once he's trained). Then upgrade all your warehouses and after that build a shield generator. Then train 2 elite honor guard and have prophet kill the people guarding the free generators. Then have your elites go in the 2 generators and then upgrade your base. Only build turrents in the front of your base and buy the 2nd upgrade for the prophet as soon as you can. Ten wait until you get enough supplies to get a scarab and then build it (this takes around 7 or 8 min if you do it right).