Get a high leveled Desmodus, land, and use its wing slash attack on the hordes. It heals your desmodus while killing the swarm. Just be careful not to fight too many because your bat doesn't have much health. Use cloak ability to get out of there if things go wrong. I did this with a lvl 60 one fighting over 10 of these guys. Just don't do it on the crazy High lvl Revenants!