Warhammer 40,000: Battlesector

For those not familiar with 40K, a Baal Predator is a tank.

The Baal Predator is unlocked as a reward for completing the 9th mission, Gallian's Staff. You can see a list of missions here. As a rough guide, I was around 8.5 hours into the game when I unlocked it.

Once it's available, you actually have to add a Baal Predator to your army to unlock the achievement. You'll no doubt be familiar with the mechanics by the time you get here, but just in case - from the campaign screen, where you select the missions, press to open Army Management and the Baal Predator will be amongst the available units on the left. You can immediately transfer it to the reserves, done on the right of the screen, if you don't want to use it (but see below).

EDIT: Off topic (and of course there are many ways to play) but I used 4 Baal Predators as the core of my army in the second half of the game and recommend them.
They can soak up a heap of damage and deal out even more and are generally effective against the larger, armoured enemies that become more common in the later missions (although not as good as Hellblasters...). Just keep the Techmarine close to repair them when needed.