After being at /warp hostile for any amount of time, at some point, shulkers are bound to teleport on top of the black wool surrounding /warp hostile. Obviously, players cannot get there, meaning all shulkers above the wool are unavailable for slaughter. Shulker teleporting behavior is only based on damage received, so once out of reach, they will no longer attempt to teleport anywhere else. With mob stacking initiating at 4 or 5 blocks, any shulkers that teleport to the underside of the black wool are likely to merge with a stack already above the arena. I am simply proposing non-solid blocks be placed above /warp hostile to prevent the teleportation of shulkers outside of the reachable area. Black carpets, bottom slabs, or any non-solid block placed above the black wool would be the ideal solution. Simple problem, simple solution, and more shulker shells for everyone!
Since it has been almost 2 months since I posted this, and absolutely nothing has happened, I'm bumping this. Would take 5 minutes to implement.