you need about 3 - 4 to make it easy...

but - before you do that - run into the tunnels and attack - attack - attack.
good way to earn credits -- (bring an engie along) Flamethrowers/Chem warriors/etc...

get a MA and turn the turret around so it's facing the short end. drive up the hill and start shelling the T-Ref. Backup if incoming fire hits you.

take a MA to the right side (same turn around turret strat) and hug the right wall -- start shelling the War Factory / Barracks.
just as you target them. Your less exposed and sometimes take them out before they know what happened.

But above all -- talk to your team and fight as one..... you will win more and enjoy it better....

Be an Tech and mine the base (search other posts for strats)--- place some mines out on the chokepoints to knock out armor (helps obi hit them harder) and stop those pesky GDI Hotwires.

Important -- use a tank/MA to BLOCK APV's and Tanks incoming. The OBI will hit them longer and you will squish the survivors.

Stealth Blackhands out on the side roads are useful for hit and run/tank stealing. (and spying).