The Sandbox Simulator allows you to simulate games against other Teams.
You can play against a list of randomly selected players in a certain elo range via the "Sandbox" tab in the main menu or you can challenge a specific opponent by clicking on their team page and pressing "Play Against". The random selection will tell you the player name, team name, current ELO rating, Division and their ranking within their division's league.
Through both methods you can select the side your team plays on.
Each day you get 25 Sandbox games to simulate, those will be reset after the daily update. You can also buy more Sandbox games in the store. The daily games will be used up first, only after, bought ones will be used.
Sandbox simulates games exactly, taking into account all relevant effects like Champion Mastery or Player Energy Level. Sandbox games do not give your players Champion Mastery, they do not cost any energy and they do not raise your Team Elo. Sandbox games do however influence hidden ELO.
Sandbox is the most effective way to test out your strategy and challenge league opponents. You can set up specific strategies to play with from the Strategy page.