Did you check which apps are using the most storage? In Application Manager, tap menu key, sort by size.
Some caches, e.g., play TV and movies, are not detected by task manager.
You can first try to uninstall apps that are not used any more. The partition has only 2GB. So, remove un-used apps.
Please try to backup your apps to external SD card use this free app: https://play.google.com/store/apps/details?id=com.koushikdutta.backup (you need install a small program on PC by following the instructions)
Then, find the apps used most space, you can try to manually check the cache and data size. For some apps, you can “clear data” to reset the app.
If the phone is rooted, you may uninstall remove some bundled apps (not so much), and re-size the partition (not an easy job).
