When preparing for this fight, I highly suggest having at least one Shock-Element Polaris Beam weapon on your person; the extremely fast DPS, especially against shields will allow you to kill the mook Guardians with ease; either to just get them off your back or to pop Second Winds as quickly as possible, especially if you are underleveled and/or fighting this battle solo. Furthermore it will allow you to easily burn through the Sentinel's shields on all phases except the Empyrean-Shock phase, meaning you can focus on dealing Health damage sooner.