To do ground combat, simply transform into ground mode. For most mechs, this is the only way to damage ground units. While on the ground, you move slower and deal less damage than in the air, so taking out a tank may take a lot longer than an enemy mech in the air. Other unique feature of ground combat are the terrain features and obstacles. To be a proficient fighter in ground mode you must make use of terrain.
Narrow chokepoints where you can bottleneck enemy units. Good for making defensive lines at the ends to take out enemy units one by one when they cross them. However, grounded mechs are also easier targets on bridges, since their mobility is limited.
You can walk down a cliff, but not back up. To get up a cliff, you must transform back into air mode and then transform back to ground mode where you want to be. Use cliffs as obstacles to the line of fire for enemy units and mechs, as well as for tactical retreats. Never have you back to a wall when you need to lift off and flee.
Use as cover in gunfights. Can use outposts, fortresses and decorative buildings for cover. Try not to get stuck in a corner in case you need to run.
Certain units and all forts have weak spots, where if damage is dealt to those areas, they will take double damage. You know you've hit a weak spot when your target glows bright red. The fort's corners and all tanks' rears are weak spots. There are currently no other units with weak spots.