Firstly HTC's Make More Space app can clear some files from "Other" by pressing "tap to schedule temp file removal", this is new to GDR2. Note that this doesn't immediately remove any files, it can't do that, it just asks the OS to remove the files which usually takes a few minutes (the app won't necessarily refresh either you should just schedule a removal then close the app, reopen it after a couple of minutes and see if it did anything). Usually only a small amount is cleared in the tens of megabytes, don't expect gigabytes. Prior to GDR2 the installers for previous versions of apps were kept on the phone even when new updates were downloaded resulting in Other storage creeping up over time. This supposedly has now been fixed too, although even in prior versions of WP8 these installers were cleared out by any OS or firmware update installation (any update that gets the phone "cogging").
Then there's all the app data which could be anything, if an app is hogging memory you won't know, since the app by app breakdown in "Make More Space" is just a list of the size of the app's program it doesn't include temporary files (the OS won't necessarily "know" that the data is temporary either because only the app really knows that hence why Make more space cannot clear it out). The only guaranteed way to clear it out would be to uninstall the app, start off with anything that accesses streaming media or heavy online content.
Then there's all the Skydrive related stuff. Basically the phone will keep a copy of everything uploaded and downloaded to SkyDrive in Other storage, this meant it was pretty much essential to turn off syncing of photos and videos to SkyDrive. I don't know if this was fixed by GDR2 but it's best to assume not. There are existing guides for how to reduce SkyDrive's Other storage footprint. Also bear in mind your people hub social media feeds are caching into Other storage ("What's New" etc).
Also I believe that syncing music can occasionally create orphaned duplicate files that end up in Other storage, not showing up on the music and video hubs. This can be a result of changing metadata or playlists on the PC. Fortunately any orphaned music and video files will still be visible through USB mass storage. So you should check what's going on there.
Basically trying to get a grip on an Other storage of that size may end up with you completely clearing out the phone (uninstalling all apps and deleting everything you can find), to the point where you may as well have hard reset the device.