He has a vulnerability frame once the yellow circle reaches the edge (full charge) and his shield dis-integrates to perform his attack. So, plant a VORTEX to hit him during this vulnerability frame. You just need Unlimited and 1-hit. Tested on Kaien and CFYOW Szayel, can't be bothered testing every character but anyone with a vortex will work.
Strong Attacks with multiple hits also work e.g. Heart Mugetsu SA2 (tested).
Last resort, spam Soul Bombs when Renji's yellow circle charge reaches the edge. The animation on your screen may not show it, but the Soul Bomb SHOULD hit him for the kill. If not, try again next attack. Least reliable method because you need to time your Soul Bomb with the yellow circle reaching the edge of the map.
p.s. I carry my main using a dummy account, which is on 1 warning after 9.4.0 (i wasn't using it 3 days before AND after 9.4.0's release, not sure when the detection was placed in the game's code). I DO NOT RECOMMEND modding on your main, on any mode.