Here's an army which seems very strong. I haven't actually tried to field it yet.
Sgt Johnson - 125 --Shotgun (blast back): Gives an extra order to 50 point unit.
Cortana - 100 -- Movable wall, very fast movement means she can take spawn portals quickly, upgrade manipulation, can trade for spartan with fuel rod gun if you need to clear off a spawn portal, especially since you won't be giving her a lot of orders during a turn.
Marine - 75 --Sniper rifle: 10 attack to start and 4 damage on doubles.
Marine - 50 -- Battle Rifle: Gets an 8 attack and grenades. If I had 2, I'd run 2, but I don't really want to buy 2 hunter packs. Synergy with Johnson
ODST - 50 -- Magnum Pistols: Full move and attack, which is nice. Grenade = good. Synergy with Johnson.