Complete the 3rd mission in the game. You are tasked with destroying warehouses with explosives, which can get a little awry. I would run into trouble trying to escape the warehouses. At the first warehouse, run back outside and find your men. The second one is a little tougher. You must plant the explosives, then turn around and go out the door one of your men will kick in. The third is simple, pretty much like the first one.
This is a good place to knock out
Just memorize when and where the enemies will run out of the warehouses at, and you can nab some quick grenade kills.