To get 3 platinum stars in later levels that have more than one entrance, cover one side with skeletons (start with 2 with a few upgrades) and the other with puddles of the beast (start with 2 or 3). Use the skeletons to generate extra cash, and use puddles to generate mana. Gradually level up each side, and spawn zombies whenever necessary. Use (constantly when you can support it) fire to weaken groups, and lightning to zap flying units. Don't upgrade the extra bonuses (except for one point in the extra demon for puddle tower) until you are all set. For the last map, I ended up with 3 mana totems, 5-6 puddles, 3 skeleton towers, 2 wealth totems, 2 cemeteries, 2 energy wells, all maxed.