Hint 1: The two harpoons are positioned to cover each other (against spiders), and both the opposite and south chokepoints. Build these first in the very first wave!
Make sure you have a strong weapon to help take down the first ogre otherwise your crystal will get damaged. You will have enough mana in the second wave to build the rest of the defenses
You can Add Monk Ensnare Auras at the chokepoints if you wish, will make things even easier but not needed with the right stats.