This occurs near the mission start. You dont need to finish the mission, just rrach the point where you are forced to exit the tank. From the start move forward and use the rocks as cover to snipe the Grunts in the area ahead with your DMR. Take out the one on the Shade Turret first. If you place your shots well you can take out the one driving the Ghost and one of the Fuel Rod ones on the dirt path to the far side of the area as well. Make sure to swap out your Assault Rifle with one of the Fuel Rods after everything is dead and stock its ammo up. When the area is clear you’ll get a waypoint down the path into the next valley. Proceed toward the waypoint (either in a Ghost or not), making sure to snipe the turret occupant on the floating platform to the left as you approach the Scorpion Tank. From here on you will want to make your way forward VERY slowly, clearing enemies at as far a distance as you can to avoid taking damage to the tank.
If the tank takes serious damage or explodes, reload your last checkpoint. If you clear the following sequences with the tank intact you will unlock Tank Beats Everything. So hop into the tank and inch your way forward along the dirt path. At each corner peak around the corner just enough to get red crosshairs on the next group of enemies and carpet the area with cannon blasts while the enemies are still outside engagement range. This avoids taking any damage at all to the tank as much as possible. You should get a checkpoint after the first couple of bends in the path and be going uphill when you see an Anti-Air gun off in the distance, with just its top poking out above the top of the hill. Stay downhill where you can just aim at the top of the AA gun and continue blasting it until its destroyed while the hill protects you from retaliation.
You’ll know the AA gun is destroyed when you hear “first target neutralized” (and it stops firing). Inch closer to it until you can see the 2nd AA gun around the cliff to the right. You still shouldn’t have any other enemies engaging you at this point. While shooting this 2nd one some Ghosts and Banshees will likely come around that bend in the path/cliff to engage you, so keep your eyes peeled and blast them as they approach before returning your attention to the AA gun. Keep in mind that you will likely have to ‘lead’ your shots on the Banshees by firing slightly ahead of them, depending on how far away they are. When you hear “Nice job Six. Falcons commence descent” you know the 2nd AA gun is destroyed. Continue clearing Banshees from the sky as your allies fly in from the left and you proceed forward.
Work your way around the next bend in the path, blasting the levitating platform on the left and any Ghosts that come in from the right. Just after the floating platform there is a large boulder in the middle of the path. Hide behind this and peak out to take shots at the Shade Turret on a small hill in the distance. Don’t expose yourself too much here as this turret’s blasts are like Fuel Rod shots and will wreck the tank quickly. I actually found it easier on my solo run to hop out and use my DMR to snipe the turret’s occupant, as well as the enemy in a 2nd turret to the right and a Fuel Rod Grunt down along the river. If your tank is severely damaged, just get out and clear the entire area on foot so that the path is clear around the building on the left and up the hill to the next waypoint. As soon as you get the objective ‘Enter Sword Base’ and still have your tank mobile, you’ll unlock the achievement.