Just got this using 2 controllers.
Have your main be the turret and the other controller be a driver ON THE SAME TEAM, otherwise it won't work.
Don't use any bots and start the game.
Now using the second controller drive through 3 green turret upgrades, and as soon as you drive through the third, it should unlock.
If it doesn't, switch to the missiles and fire at the ground, if necessary.