This spell requires a reaction. It is, therefore, best to use it when you are faced with powerful elemental attacks. If you are unable to take an elemental hit reliably and fail the saving throw to reduce the damage or to half, you should try to absorb as much as you can.  The spell states that you trigger the spell when you take damage. You can still roll your saving throws to prevent the damage from being done before casting it. This is a great way to save half of the damage by using Absorb. You can get half the damage back with Elements. This spell can be used to make up the difference if you aren't confident with your dice rolls and your HP.