Do this on the first map. Add some new shrines by pausing

selecting shrine over the location desired

and putting it down. Hopefully your maxed out with the blue hero and he can boost your heroes from a distance. To send a wave early you must pause

select the next wave and press

but 2 things you need to know. You can't call 2 waves in early. It is important to wait until the wave gets close to dead, especially the slow ones, because if you kill the second wave before the first then you will fail automatically when the third wave appears naturally. Second thing to note is that there are faster/slower waves. Calling in a wave with a grass hopper when a slug wave is out is definitely going to cause you some grief. It may take a few tries but it should be more than doable if you slaughtered insane mode to begin with.