Description says it all. You just have to destroy a tower before you reach 8:00 minutes on the match clock. (which is in the top-center of your screen)
(Found this easy to do in Conquest while playing with He-Bo and pushing right lane with Medusa.)
Tip: If you look at your map you can see the health of a tower and may be easier to destroy that one tower as a group to strongly push.
Tip: Let the AI bots go into the tower first so that they can take all the damage while you guys focus on the tower.
Tip: Stick with basic attacks while hitting the tower, it's easier to do than using your abilities.
Hope this helped! And good luck!