You don't have to stack up for Shriek... there is a huge arena to move around in and stay apart from each other. We have our 2 healers and 2 of our 3 ranged sit in the middle circle(or just outside it during poison) nicely spaced so flame circles never double hit, our last ranged is kiting and stays to the outside and the tanks/melee just move on the outside circle. People don't have to run around with their heads cut off, four of us barely move during the fight, and really only move if people are close and we need more space to get rid of voice.
Super shitty 5m pic.
Purple is Melusine, Blue are tanks, Green healers, Red is DPS. We have 3 ranged, 1 melee DPS. Kiter on the outside moves as needed, healers move to petrify as needed, tanks/melee move as needed based on where Renaud is, but the 2 Ranged in the middle never really have to move and thus, the other Ranged DPS just stays away from them and you never, ever, ever, have to worry about double flame circle unless someone is dumb and runs in between the two ranged in the middle.
We try to keep the Renaud on the back outside circle, but occasionally due to shitty spawns the tanks have to move pretty far, but they just wait until 2-3s left on shriek or circle blade and then bolt for cover, so Melusine doesn't rape anyone with tail slap.