The whole goal of therapy in a dissecting aneurysm is reduction of aortic wall tension, which requires reduction of both HR and MAP.
Analgesia alone is likely not enough, though fentanyl in large enough doses is a good sympatholytic.
Labetolol is an OK choice. Esmolol is better, IMO. Nitrates are fine but are usually given with a beta blocker in order to reduce chronotropy.
There are different protocols out there, but a common goal is an SBP of 120 or less and a HR 60-70.
I once transported a patient with a dissection who had been intubated so that he could go on a propofol drip because of intractable pain and hypertension. I don't remember what all they tried before the propofol, but he was on a large dose of it as well as nitrates and beta blockers.