So I was rereading Volume 9 just a while ago, and noticed that, when Barry and Falman were being attacked, Ran Fan said there were 10 to 20 people outside, but none of them were moving. Of course there weren't that many outside, only Envy and Gluttony.
Then I remembered how Ran Fan and Ling can sense the presence of the Homunculi because they have a strange aura, but they couldn't detect Wrath because he had an aura of a normal human.
And then I came to Ran Fan's question to Envy: "How many are in you?"
In conclusion, we can assume that the Homunculi have the auras of several people each. I think this is because their hearts are made of the Philosopher's Stone, and to create that, many lives are sacrificed. Thus, when the Philosopher's Stone runs out, so do the number of 'people' in their auras. So maybe just before Lust died, her aura became that of only 'one.'
And so here's my theory: Have we ever seen Bradley die? We've seen Lust, Gluttony, and Envy killed dozens of times, but never Wrath. Maybe this means that he only has one life, so to speak. Kill him once, and he's dead. And so far he's done a great job in avoiding death; he's too fast, he's too powerful (even Greed was no match for him). So perhaps this is the way he will be defeated: dismembered alive, and since he can't come back from the dead unlike the others, that will be the end of him.
Now all we need is someone powerful and clever enough to do the job. (Roy? Ed? Ling? Father? The other Homunculi?)