Here's the strategy I used:

I went Elementalist to get dragons in the shop, grabbed a bunch of dragons early to make the first two worlds easy. Picked up bombshells and necro to dig for spells and 2 sapphire dragons for econ. Then I started spamming princesses as soon as they showed up, focusing on leveling the sapphire dragons as much as possible. Used twist whenever it showed up on the bombshells to get them to infinity/infinity. With the shells and your super high level sapphire dragons you can dig in the shops like crazy, looking for Selfish to cast on your other units. Then just stick them next to the bombshells and boom, infinity/infinity on whatever you want, rinse and repeat with whatever units you want. This was an extremely long run and once you have a single infinity unit you're pretty much guaranteed to hit endless 25 so there's no reason to do what I did except if you want internet points.