Overgrown cats won't work as they do not hunt rats and a kitten is not recommended as the chance of them catching a rat is a low 15 percent compared to 50 percent for a normal cat, and will take longer to complete
Up to 9 bowls of stew (If you make no mistakes, you will not need more than 9 bowls, following the instructions below. The Forester's Arms sells stew for 20 coins, which may be more convenient than the Grand Exchange (637 coins)).
Bucket of milk (Only if you wish to change your hellcat back to a normal cat)
Varrock teleport runes, charged amulet of glory, for quick transportation to Edgeville, or the wilderness sword. Alternatively, lodestones can be used.
This method is sort of like a game of Mastermind. The best strategy is to have lots of stew in the inventory (9 is the maximum players will need, barring mistakes). Have a cat catch hellrats to get spices. Spices can be red, yellow, brown, or orange, and may have from 1 to 4 doses per shaker caught. The perfect stew will have between 1 and 3 doses of each of the four spice colours.
Figure out each colour separately. Use red as an example: Add one dose of red spice to a stew, and then ask Dave to eat it. If he says that one spice is right, stop and move on to the next colour. It will then be known that one dose of red spice will be needed. If he says that no spices are right, add two doses of red spice to the next stew. If two doses are correct, stop and move on to the next colour. If no spices are right, the answer must be three doses. Do not try out three spices, because using this method, players now know red must be three. Move on to another colour—starting with a single dose of that colour—and repeat this process through all four colours.
After figuring out how much of each colour to put in the stew, use that much of each colour on one stew. This will be the "totally evil!" stew.
After determining the right combination, have Evil Dave taste it and finish the dialogue.