In the Challenge mode, missions are completed in sets of 6. You must complete all missions in a set before you can advance to the next round.

If you don’t earn all 3 stars on a mission, you can replay it with no cooldown as many times as you want to get the missing stars. However, you can’t go back to the previous round once you advance to the next one.

Getting stars will unlock special crates such the Class Token Crate and Hero Token Crate.

Each time you complete a set of missions , their difficulty increases. The more difficult set you complete, the better rewards you receive.

Your starting difficulty is based on the average level of your top three highest survivors.