Historically speaking, Ret comps don't perform too well versus caster cleaves. So there isn't a whole lot you can do, but, try the following stuff;
I would advise you to save your Freedom for when the Boomkin has Solar Beam and is about to Root-Beam your healer. Keep in mind it has a 1 min cd normally, and a 45 seconds one if it successfully interrupts a cast. If Boomkin doesn't have Beam up, then you can use Freedom in combination with your Horse under one of the following situations;
- When you are going in for a HoJ on the enemy healer.
- When Ele has already used Thunderstorm and you need to get back to your target.
Having Boomkin be your main target is probably far easier than trying to chase the Elemental Shaman, especially if the map you're fighting them at is Tol'viron or Blade's Edge. Both of them can kite you pretty well, so I would also spec into Divine Favor instead of Crusade and then probably Word of Glory (since you won't be in melee range too often to make effective use of Justicar's Vengeance). Don't spec into Law and Order, unless in maps where you know you will be able to melee alot - which again, if they know how to play, should never be the case.
I would also advise you to always run back and LoS the Boomkin when he uses Incarnation. That damage is way too retarded and overpowered to tank in the face, so you want to be avoiding as much of it as you can (which feels funny to even type when said shit lasts 30 goddamn seconds). As such, I personally pop Wings only after the Boomkin has popped Incarn and I am on dr with some of their cc.
I would say you need to provide extra info if you want further help though. What comp(s) are you usually playing? What's the exp of you and your partners?