A good strategy that seems to work ,for me with the UNSC is,for deathmatch skirmish mode,build four supply pads at your base and 1 air pad with a vehichle depot and a barracks. Train 3 additional warthogs,take over an additional base build the same buildings and you will have a total of 25 population. With each base u gain 10 population. from that point on each base you claim, build all supply pads for increased supplies and build whatever units u want from your other 2 bases and you will dominate.
REMEMBER:BUILD VULTURES, HORNETS, AND TANKS OR THE ENEMY WILL DESTROY YOUR ARMY. ALSO TRAIN FOOT SOLDIERS TOO...