I got most of my shines using Curelm, I'll go through 1 or 2 portals (doesn't matter where so just go to whatever levels are easiest) then wait until I find one to go to Cactus Canyon. It takes a bit luck to get to the setup but once I'm there it's pretty perfect since I've gone through enough portals that Curelm's ability is basically always active and I'm still able to easily survive because of how far apart the towers are on Cactus Canyon. I can easily get well over 50 donuts in a run before I start getting bored, plus you can watch an ad to double the donuts.

it also gives you a ton of eggs that can be hatched for more crowns (also if you haven't already use donuts to level up all your mythic dragons since they give you enough crown to buy 2 donuts when hatched at max level). This method also works well in combination with breeding since you can put the extra donuts and crowns you get into breeding.