There are several potential factors that could greatly reduce your battery life. The display uses the most battery power, so to start it is recommended that you turn down the brightness. Also check for any applications that you are not using, but are still running in the background. Close these applications. If you are not using WIFI, you can turn it off so your phone is not constantly searching for a signal. After making these changes, if your battery still drains quickly, then you may simply have to replace the battery.

Have you try using our troubleshooting guide? ASUS PadFone X Troubleshooting

You should first check to see that you have a strong WIFI signal. If you have verified this, most likely you need to edit your WIFI settings on the phone. Swipe the top of the screen down, and click on the settings button that now appears in the upper right corner. Select 'WIFI' and then choose 'Advanced'. Uncheck the option for the WIFI to disconnect when the phone enters sleep mode. This should solve your issue and prevent your signal from consistently dropping.