Oh, I think I have just determined "the plan".
Of course there is no way for me to prove that I am correct.
For if I say it here then Lightfoot could modify the story and I would not want to cause that.
Suffice to say every good superhero has a Achilles heel. Creative exploitation/compensation of that weakness either by a foe or the hero makes for good stories.
One could almost make a flowchart of "What to do if..." for superheros.
If presented with foe of type X then counter with action Y, etc.
That action is typically based on the exploitation of an inherent weakness that is not readily apparent.
As with the current antagonist - a mimic / metamorphic character...
For a MIMIC / METAMORPHIC character
The typical counters are:
1) Prevent the mimic from using their ability, e.g. prevent them touching you or getting a new shape (i.e. Rouge), render them **61** (i.e. Mystique), create a situation where changing actually does not help or makes things worse (i.e. Beastboy, Morph), etc.
2) Overwhelm or otherwise saturate them with new ability they can not control or understand how to use, e.g. the opposite of Option 1 (This has been used with Rouge, Warlock, Wildcard, etc.)
3) Exploit a "hidden" weakness in the current form taken or power being copied.
There are surprising number of shape shifting characters out there and almost as many counters to them. Coming up with the counter when dealing with a doppelganger actually provides insight into the person or power being copied. When handled well it can be a good metaphor to move the story along or indirectly deal with issues that the actual character could or would not.
In Pulse I wonder what power Annie has that another might not be able to control or be able to compensate for?
On of the best exploits of Option 3 was used in the conclusion of Disney's Aladdin.