To unlock the Totsiens achievement in Ghost Recon Breakpoint, you'll only need to finish the main story mission "The Great Escape." With three scientists in tow, you'll take command of a minigun on the back of a military vehicle, shooting enemies off the roads and out of the sky along the way.
After about five minutes of driving and shooting, you'll come to a bridge where the objective changes. Here you'll need to kill all enemies on the bridge, but this will extend to another two helicopters, including one in which the level's boss sits.
Her chopper is reinforced and takes a lot of damage. Try to get the people on the bridge as fast as you can, making good use of the ample red barrels lying about rather conveniently. Then take out the lesser of the two choppers. More enemies will arrive by vehicle too, so watch out for them.
Eventually you want to get to a one on one deathmatch between you (or your squad) versus the reinforced helicopter. To make the quickest work of the chopper, use the miniguns on the backs of the now several trucks parked nearby. There should be at least two, and though she'll eventually destroy them while you fire back at her, the miniguns do great damage on the chopper and after two to three vehicle swaps and minigun firestorms, you'll see her chopper catch fire and crash below, completing the mission and netting you the achievement.