[@Natascha] There is a way to do it with showdown actually... Kinda...
You could open up the simulator in two different browsers, create your team and the opposing team, and play against yourself by manually inputting each attack in the round. 

However, that may be more work than its worth for one person. Also, relying on a simulator limits the tactical options you have during roleplaying, such as using terrain to your advantage or things like that.