Well, it's kinda obvious for me, but i can't be sure about other people, so i decided to post this.
When you take quest for bombing, each day you have maximum 3 of required mobs to bomb, but if someone invite you through cross-realm, you'll see different setup of this mobs. Just pass through several realms with this quest and achievement is yours! And do not bomb normal mobs, because you'll not be able to fly again if you complete quest himself (or you can just abandon it and take again).