1. A "good" Vampire Guardian Angel (there's only one left vs. thousands), or a Vampire Guardian Angel who has turned back to good.
2. As long as the "creator" Ms. Price is alive, nothing can kill the Vampire Guardian Angels as they can instantly heal or be healed by her. But there's a catch: If one kills the creator, the only angels who die are the ones who have had a blood pact with her (which explains why Ms. Price did not die when the Vampire Guardian Angel Zeke died-- they never made a blood pact. In the novel, he forcibly took her, but in comic book Issue 3, it was only implied.) And to date, there's only less than 5 who have (or who will make) the blood pact.
3. The blood of a "good" angel on a weapon. In issues 3 and 6, for instance, the "good" angel Cameron smeared his blood on his sword and on Detective Costa's machete, and in both instances, Costa was able to kill the Vampire Guardian Angel Zeke and one of the Angelorumrabidi.