Just to clarify: if you play like I do, you will probably get this eventually on the campaign. (When I play, there's more led in the air than in Chuck Norris' diet or in a Chinese paint can.)
However, if you do not earn this in the campaign, a very easy location is in chapter IV, checkpoint 'Across the bridge'. Choose easy difficulty and character Breakdown. Proceed for about 15 seconds and you will encounter approximately 10 cloakers. Make sure you have your Scatter blaster equipped and start scatter blasting! Try to aim at head height. If you run out of ammo, there's more on the hallway. If you run out of cloakers, just press start and restart from the last checkpoint.