in the campaign once you have defeated hall of court on survival until wave 15 I recommend doing it on medium but easy will be okay, you would of have to of also defeated foundries and forges on insane to of had the right stats. To complete this level you should have your stats in three stats, tower attack = 150, tower range = 50 and tower attack rate = 130 or how many points you have left over.
Wave 1, you only have enough mana to place two bowling ball turrets one in the middle and one on one of the other sides makes sure they are facing inwards if you don't no what I mean by the chests, switch to your dps with the genie.
wave 2, you drop your mana to your tower squire build 3 more bowling ball turrets facing inwards in the middle on the two sides build 2 bowling balls facing each other and then 1 harpoons next to the side bowling balls.