Just for reference for those who haven't played Tera, here is a very basic overview of Surjuka Gallery. The goal is to defend a crystal in the center of an arena against waves of enemies. As you play, your group earns points and your groups score at the end determines how good your reward is.

Some examples of ways to get points:
-killing with basic attack
-Each mob killed is worth a certain amount
-Remaining health on the crystal you are trying to protect
-Time remaining at the end of the round
-ect.

At the end of the dungeon, if the group score above X they would get a gold which gave the most tokens, if they got above Y, the would get a silver and get a couple tokens, and if they get above Z they get a bronze for very few tokens.