You find the summoner by opening presents that you get from killing mobs. It's called the Snow Globe and it has around a 6% drop chance from those presents. If you have a mob grinder, it should be easy to get.
EDIT:
Killing the Frost Legion shouldn't be too hard, if you have a proper arena where they can spawn. Doing it just in your town will take a freaking while since the spawn rates are immensely low. The mobs themself should be no trouble to you with Hallow armor or better.