I was also facing the battery drain on IPhone 6s on 11.2.2 and later on 11.2.5.
If you can check which apps are utilising the battery with background processes, you can definitely stop the battery drain.
For me the Facebook app was the culprit. The solution is very simple. Delete the app.
Reboot the phone and take a backup with iTunes.
And then restore your iPhone. You can always setup the phone as new or restore the backup.
Make sure you remove the battery draining app before initiating a backup.