A new online service Don't Kill Your Party aims to simulate encounters in order to create a suitable challenge level to match the party:
The site is currently accepting alpha testers:
https://www.dontkillyourparty.com/alpha/signup
We built this encounter simulator to help the wary Dungeon Master avoid situations where that encounter they made which was classified as easy by the DMG ends up wiping the floor with their poor party members.
The site is currently accepting alpha testers:
https://www.dontkillyourparty.com/alpha/signup