The above solutions are top notch, but thought I'd just suggest another alternative
If you head out into the middle of Panau Tengah Bay you'll find a circular shaped military harbour (Pulau Berapi). It's pretty hard to miss. It's massive.
On the bottom left hand side (if approaching from the south) you'll find a helipad and a collection of one level buildings beside it. A Stonewall vehicle is amongst these buildings. Jump in and let the slaughter begin! Soldiers spawn like crazy round this area so wait for 'em to build up and grab that achievement! I got forty kills within a minute here.
It should be noted this was done while the island was incomplete. I don't know if they'd still spawn as heavily if you had already completed the place.
Also, this is a pretty good place to get the 'Invincible Warrior' 'chieve. Spawning enemies and lots of cover equals easy achievement. Remember to get out the vehicle if going for the latter though.