Though some may have found this obvious,
this does NOT need to be done Online.
While the Online kills do count towards the 100,
the best way to unlock 'Big Guns' is in the Single Player
('Enlist Now' on the main menu)
One of the first few missions should be called 'Big Guns' (Go figure)
On this level you learn how to use the turret, and thus is great for earning the achievement.
Regardless of difficulty, one playthrough will net you many Turret kills.
(Harder difficulty=Many more kills)
Finish once, repeat till Achievement pops.