Because the A37f has a locked preloader, we use to flash TWRP.
The (often the Asian/Indian variant) was released with ColorOS 3.0 based on Android 6.0 Marshmallow .
Even a top ROM can suffer from battery drain. Greenify (root mode) hibernates background apps. With Android 6.0’s Doze mode, this combo yields 6+ hours SOT on the A37f.