There really are no secrets for this game. Take advantage of your color spread fiends (in this case, Gene and Bob, if you have them), or just make your other fiends as strong as possible, beat the slug with as many large matches as you can, then get the requirements after he's dead. Find a level that you can beat easily that will give you yellow and blue mites and just beat it over and over until you feel confident.

Outside of that, it's really just luck. A matter of doing it until you get a lucky break. I got one after about 6-7 tries, so it's very doable.