I made the smooth terrain tools work in-game so it is easy for developers to allow users create with terrain in-game.
You can find an example place here.
Just drag the model into StarterGui and it will just work, even on mobile!
You can also find the code on the Studio Tools GitHub repository, the same code used for the in-game version of the tools can be used for the studio version.
I wanted to make a smooth terrain building server a long time ago, but I could never find actual smooth-terrain tools that worked in-game. 
Region tool is nice for removing everything.
Needs FilteringEnabled.
[quote]
Region tool is nice for removing everything.
Needs FilteringEnabled. [/quote]
I don’t know, I thought about what I’d expect from the tools as a developer and I think the expected behavior is that the terrain is edited locally when filtering enabled is on. It is pretty easy to make a system with a RemoteEvent though to replicate the changes if you want to do that. (You’d just be replicating all the WriteVoxels calls)