When playing as the covenant, ensure to play as the Prophet of Regret in a 3v3. In the game, build 2 warehouses, a temple and another warehouse. When you have 100Cr, build another warehouse. When you reach 400Cr, purchase the Prophet's Blessed Immolation. Immediately send him into the nearest rebel/flood base/reactor, and DO NOT use the Cleansing Beam, unless his shield drops below half before you have killed most of the rebels/flood. Do this 3/4 times and he will gain 2 stars as long as you DO NOT use the Cleansing Beam. If there are forerunner reactors, use an Elite Honour Guard to claim one, and get Age of Doubt when available. On all claimed extra bases, build a sheild generator and 2 warehouses. then upgrade to Keep and build a further 2 warehouses.
On your main base, you should by now have upgraded to Blessed Warehouses, Citadel and built a Shield Generator and a Summit. On extra bases, upgrade to Citadel and build two Summits. Save up, using teammates and turrets for cover, then get Large turrets, full Prophet upgrades and Followers (when you get Tech level 2, get Ancestral Perversion and Forerunner Shield before the first Cleansing Beam upgrade), Then fully upgrade engineers and build 30 of them, as well as a SCARAB heavy assault platform (if using an Elite to claim reactors, build 29 Engineers, ensure all Tech level 3 upgrades are purchased, then send him into a base/army to die - then build the final engineer, and eventually upgrade to Age of Reclamation).
Excellent for both defence and attack, ensure that the engineers always follow the scarab, and use the scarab to prioritise anti-air units (vampires and wolverines), followed by anti-armour units, also dispatched by the flying Prophet (cobra, hunters) before attacking turrets and other units. The 30 engineers will allow the scarab to survive a quadruple MAC blast, whilst being attacked by wolverines and grizzly tanks, so it is extremely necessary to eliminate wolverines and vampires.
See http://www.halowars.com/stats/StatsGame.aspx?Id=97823549-2057-2327-0810-000000000000 for the potential results - but always ensure to go on the OFFensive with teammates, to ensure scarab survival - not so necessary on defensive. A handful of cobras, wolverines, a flying prophet and a 30 engineer/scarab combo make for an inpenetrable defensive line. This tactic is especially effective on a larger map such as Fort Deen. Much less effective on Exile unless both of your teammates attack at the same time due to the ease of a rush, and fewer defensive options. Could work well on Frozen Valley, due to large avenues of assault.