This one is pretty easy to do
The Grunt/ Jackal Flank
You obviously need to be playing as the Covenant. When you start off, you need to build 3 Warehouses and a Temple. While your buildings are being built, you should send your hero (preferably the Prophet of Regret) to scout out for resources. When your 4 buildings are done, you should upgrade to the Citadel and make your prophet and ghost both attack the rebels at a Forerunner supply station. When all rebels are defeated, go back to your base and build a Hall and a Shield Generator. When both are done, train 2 groups of Grunts. Send one group to the Forerunner supply station and send the other group to a floating tower near your base. After that, you should train 6 more squads of Grunts and upgrade them with the Peon upgrade. Afterwards, you build 12 squads of Grunts and 8 squads of Jackals (making sure that the Jackals are upgraded with defense gauntlets). When you are done, set the global rally point at your enemy’s base and charge all of your Grunts. While your Grunts are taking all of the brute force, send your Jackals around to the base of the base and have them attack.
This is very easy to do because I’ve tried it several times on Heroic and it worked perfectly.