Oh boy the middle tier endboss being significantly harder than the one of the first tier... that has to be a first? Oh wait yogg+0, ragnaros, lei shen.
The truth is guldan was actually too easy for an enboss and that was only covered by the fact that it was a 15 minute fight with a new phase right at the end. Mechanically it honestly wasn't much harder than pre nerf/change star augur/elisande.