I mean seriously if a player using the Covenant doesn't leader rush you all the do is build a scarab and put a -blam!- ton of engineers on it. Then the scarab is invincible. If you try to kill the engineers then the scarab kills you while the engineers heal themselves. Even using super carpet bomb, super MAC, and super cryo doesn't kill them. There needs to be a limit on the number of engineers each person can make. Maybe 10 at a time. Discuss Halo Wars ITT.
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Early on in the game, send a few units over to their base and destroy their temple. Rinse and repeat as long as you need to; just keep your strike-force close-by the base and attack every few minutes. Make sure the units can get in and out quick, too.