A handy tip for Mission 5: Arcadia Outskirts.
Turn on the skull that gives a 5% chance for non-player units to explode. It can easily wipe out a grouping of infantry in the early part of the mission, which is a godsend considering the sheer numbers that get thrown at you. I had the Spartan Laser using guy garrisoned in the overturned Warthog to the side of the broken down highway by the crater. He would laser a group and the explosion would take out 2-3 separate squads.
Once Omega Team shows up, the mission is essentially done. Take out the remaining attackers, head on over to the Mega Turret, build the second base and get some Vultures to take out the Covenant base.
Of course, the problem is surviving to get to the point where Omega Team shows up. I can't believe that both this Mission and Mission 4 made it through QA.