Welcome to all the folks who found this game via Nerd^Cubed! This was an entry for Ludum Dare, so it was made in 48 hours and not super polished. I'll be releasing an updated and expanded version soon so keep an eye out. Thanks!
Here's my 3 star strategy for the last level: for traps, I had 3 rows of fully upgraded archers in the back, 3 rows of fully upgraded barriers and then 3 rows of maxed out freeze traps. My weapons were maxed out arrows and lightning spell with max mana upgrades. Basically I let the archers take care of most of what happens on the ground while I use lightning and arrows to kill drakes and replace barriers (healing uses too much time / mana) as necessary. I try to zap orcs and elementals with lightning to soften them up if I can and when the skies are clear I just rain arrows into the crowd.
Guys I loved this game! I can't wait until you get more money/sponsorship and get a bigger version out. In fact I'd probably buy it if you did a full Steam downloaded version for $5 or $10 or whatever is reasonable.
Guide for infinite time on survival: lvl 3 lightning, reaper, soul jar, locust, polymorph and earthquake (my earthquake is actually level 2). use earthquake whenever possible and make sure you always have a reaper summoned. always use soul jar whenever you can get more than 3 enemies in its path for mana. once you get to levels 6/7 that's when you can have infinite playtime. soul jar can hit 10+ enemies if you fire it in the center when enemies are bunched up, and then you can lightning the crap out of them once you get mana from them. sun kings are not actually that problematic. the reaper is amazing at killing the sun king. polymorph is your main weapon against the grunts and champions (which are the real threats). if you have earthquake ready use it before you polymorph. if your reaper is dead lightning and locust the top , polymorph the top once and then resummon the reaper. healers aren't that much of an issue since your lightning and locusts can kill them pretty easily.
For gem levels you can use explosive weapons to push gems around. The Browning is the best IMO. I just push all the gems into a pile and then kill everything around them as they come in.
Also for the Just Do It list: anvil on nuke, lightning on nuke, napalm on nuke, dart on bullseye of dartboard level, and kill lots of zombies with the basketball on the ball court level.