Play as the Covenant, preferably The Prophet Of Regret, set your opponent to be Captain Cutter. Set the opponent AI to Easy to make it easier for you.
Build supply pads only, upgrading them, and a temple, and upgrade your ages (upgrade found at the Temple, Age of Doubt first and then the Age of Reclamation) so you get tech level (the lightning bolt in the top-left corner) to three - this will require two ''Age'' upgrades. Also, capturing a second base is almost mandatory and increasing your population (this upgrade can be found in the Temple as well) helps too.
Kill any unit you might get using the Prophet's Cleansing Beam if your population is over 20. Make sure you have enough population room and enough supply to ''Buy'' a Scarab. I know you can't buy units, but you need the supplies. So have 4,000 supplies in reserve and 20 population available.
Then just hope you get a Scarab instead of a lot of Wraiths or anything else. If you have too many units and they take up too much population, the Prophet's Cleansing Beam is your friend. Kill your own units with it if needed.