Requiring that a non-rotation guild beat a rotation-guild to [mob x] to earn a rotation slot is bad system design. the rotation-guild is already at an inherent advantage because it can dedicate resources to the specific gatekeeper mobs without having to bother competing for other, also rotated, mobs.
IMO the only fair way to manage the rotation is to have some other metric, like "can kill [mob x] solo or in [y amount of time]." Let guilds request and be given the opportunity, to get the kill on whichever is the gatekeeper mob. If they fail, some kind of cool-down before they can request again. Maybe add some other function whereby a guild can lose its rotation spot if it fails to achieve some basic result, and then also has to sit on cool-down for a few weeks before getting back in.
Importantly, this needs to be a GM enforced meta, though. If it's just a player agreement, all it takes is one guild to say "no, we're not agreeing to that. we will consider all mobs FFA," to ruin it.
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