This does not have to be five at one time, just five in total. You'll want to stock your character with a good amount of Molotovs and either Grenades or something like a Deo-Bomb. Throw down a meat bait, then a Molotov to set a group of enemies on fire, then quickly switch to the bomb and throw that as well to blow up the flaming group. You can also put the meat bait next to an exploding barrel and shoot it rather than trying to switch to a grenade. In fact, the flames from the Molotov will likely explode the barrel on its own. Depending on the type of enemy and their level, you'll need to be quick before they die from the flames. You do not actually have to have personally set them on fire for this to count, just be the one to blow them up. Some enemies spawn on fire; those will count for this as well.