If you haven't got this by the end of the campaign or are looking to knock it out quickly, load up level 4 on 1 star. You are supplied with 3 grenades from the start. Just kite the enemies for a bit and then lob a grenade just in front of them. Repeat with the remaining grenades and kill any stragglers to finish the level. Repeat the level as necessary - I needed two runs after the campaign.