A solo-tank prot paladin using Seal of Insight and Battle Healer = bats will immediately go for him. At this point, you will want the paladin to drop a consecrate/light's hammer and immediately HotR the bats. This tank needs to keep Tortos focus-targeted so that he can see when the boss casts his Bite attack (failure to do so will usually result in the bats getting healed and the tank possibly dying).
As soon as the prot paladin hits HotR on the bats (keep in mind that he actually has to be targeting the bats. If he's targeting Tortos, the hammer won't hit the bats), your DPS should be able to start. From this point on, the tank should pop a minor cooldown and reposition to guarantee that the bats are not hitting his back.
Recap: Focus target the boss. RF + Seal of Insight + Battle Healer = bats go straight for the tank. Drop consecrate/light's hammer so that the bats will hit it before/when they reach the tank. Immediately use Hammer of the Righteous on the bats when they arrive. Reposition so the bats are all in front of the tank. Kill the bats. Make sure to use Shield of the Righteous any time Tortos casts Snapping Bite. Oh, and avoid the circles on the ground.
That's literally all there is to it. If your DPS can hold off long enough for the tank to HotR, they shouldn't pull aggro. If your tank can focus target the boss for ShoR purposes and not get hit in the back, damage shouldn't be an issue. It's pretty easy, especially with a few attempts of practice.
Warning: If you aren't solo-tanking, do not allow your prot paladin to tank the boss with Seal of Insight... your other tank will likely have major issues acquiring the bats.