Final Shape campaign last boss issue

I'm running through the campaign solo and have gotten to the fight with the Witness at the ritual site.

I'm having an issue during one phase where the light shield just won't stay up. The first time this happened I thought it was because I had run out of the protection from darkness, so I tried again and made sure to get that buff back before the arena-wide attack procced.

However this did not help.

I get the strand subjugator down to half health and go get the shield, attack his shield, and prepare for the AOE attack from the Witness. I hold down C (I'm on PC) and my Titan puts the shield up for a few seconds, before immediately letting his guard down and then taking huge amounts of damage before I can put it back up again (only for a few seconds again), repeat until I am dead. I am not sure what I'm doing wrong. Is this a bug? I definitely have the shielded from darkness buff, I'm avoiding damage from the subjugator, the shield just wont stay up. Any advice appreciated.