Given serenity just means long-term peace of mind, then I don't think you can be in constant state of serenity. You can be most of the time in this state, but there will be moments where you will feel anger, sadness, etc. And how to achieve this? Void meditation for calm mind and just meditating in general so you will be at higher vibrations and will be able to look at things with higher consciousness and don't have unnecessary reactions.