How do I "End a round in under 10 seconds"? What's it mean by "round"?
A round is a the whole level. A round starts when you enter "in-game", flying over the play area. Typically a round ends when you can no longer defend the base from zombies.
The only way to end a round that quickly (10 seconds) is to intentionally fail. The easiest way (only way?) to do this is to kill civilians. You'll likely have to kill three civilians in one shot.
Additionally, remember you can always use coins to bypass these challenges if you're having too much trouble with them.