Amara shields

I have been farming various bosses for some time now and regularly look into the Dr meds for the one shotter shield but never seen one. Today I found a triple roid berserker shield with the anniontment after using phaseslam melee damage is increased by 200% for a short time. It seems to be better than my brawler ward using the ase melee damage is increased by100%.

when I use these shields with phase grasp the berserker is out performing the brawler (300% melee) and my revolter (2× absorb)

Can anyone tell me why? is this shield rare(berserker)?.