Just got through soloing Marracco, and with that my revenge against the dragon who killed me while I was questing is complete.
I periodically checked Coerthas for signs of him and finally found him and killed him uninterrupted.

If someone says in shout or LS "Hey there's this A rank mob here" then you should probably wait since you're not the one who found the mob, though if you stumble across an A rank or S rank then you have every right not to say anything about its location and kill it. (Especially if you solo an S rank, if you do that then I'll be impressed)

But if you're really that desperate for A rank mobs you should be desperately running around trying to locate them before anyone else.