Troubleshooting these things can be a bit tricky, as battery life depends on a bunch of different factors. Some general tips include rebooting your phone regularly, turning on Super Clean mode, preventing applications from auto-starting, cleaning up applications in the background, preventing background sync, and so on.
A lot of times there's an application going haywire in the background without you knowing it. Finding out which one might require some detective work if a factory reset is off the table.
If you feel like your battery might have degraded, put it in flight mode before going to bed (mobile data & wi-fi disabled). If you've lost more than ~8% after a full night's sleep there might be something else going on that we'd like to investigate further.
