To counter marine vehicles ,the Onos should be able to have a gorge build DCs, MCs or SCs on top of the Onos.like the Overlord Tank in C&C: Generals.
Then we would need an anti-vehicle lifeform like the Zerg Lurker, which could burrow and use underground spikes to flip Vehicles or cause serious damage. That could be countered with Grenade launchers.