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[AntarcticTiger]

Try setting your vehicles to either Cautious or Evasive tactics. That way, they'll retreat once their constitution has been lowered a certain amount.

Stay at high altitude, and try to make use of the Justice/Retribution heavy missiles, or plasma weapons, if you've got elerium. I managed to fend off most alien attacks with a force mostly consisting of hoverbikes, until the battleships and motherships showed up.