When you choose your method of playing, there's something at the bottom of the screen that says "eat your greens". Are you promoting us to be healthy Edit?
My way of getting the gold medal on level 20 is by doing the following: Start with the turtle, followed by a goblin, then a golem, healer, goblin, elemental (since you'll be needing to use your spells a lot), 2 goblins, golem, healer, and finally 2 goblins. A few tips to keep in mind: Always stun the bog tower. That way, all the cannon and archer towers will go for the turtle. Fully upgrade everything, especially the Quake Spell. Use the Quake Spell when you're about at wave 3 or 4. Heal anything that has about 1/4 health left. Since you fully upgraded it to have 400 health, it can really change everything. And that's about it. + this to keep this alive for others, please!