did we ever find out if stacking the seeds would work? I understand it would be hard to position the players in exactly the same place, but it might work if all players effected ran into a specific corner so the terrain actually forced them into standing at the same spot.
The reason I bring it up is for the final phase we seem to have 1 extra seed and 1 extra pillar both at the same time, if we soak the extra pillar then we run very short on players who can soak the waves.
Instead we could ignore the last lot of pillars (or maybe soak 1 or 2 that are quite far from the other pillars), have the people with seeds stand in the same corner and then have the players with mobile magic immunity (rogues+clos, paladins+bubble) absorb all the waves from the seeds as they spawn from the same place.