For that you need to press the two swords at the bottom left of the screen and start a multiplayer game. For each match you must have bought your units and place them at the beginning of the battle.
A game is considered won if you get to destroy the dungeon before the time runs out, i recommend that you first destroy the archer towers before you tackle the dungeon.
After this, you'll be able to destroy the rest of the buildings for a 100% destuction score.
Just repeat the process 20 times.