Okay, this thread is majorly spammed up since no one has actually addressed the question of this topic, which is level 18.
Silver7, here's a few tips in case you still can't beat that level. You only need two towers since you've got two colors of gems. Build them in the two tiles directly to the left of the shrine, inside the loop. Naturally you'll have to destroy the infested building that's between your gem towers and your wizard tower. It only has 900 hit points, so it should only take six gem bombs even without any gem bomb skills. With Forge level 8, you can destroy that building for only 138 mana.