Observations

At this point, I have to assume Mat shan't be helping. Apparently, people don't much like it when you make it sound like you know more about their friends' deaths than they do. Nor do they act any less hostile when you then imply that they were acting stupid.

Regardless, I need to push on with my theories. Even if I do have some form of shielding against its manifestations, it'll find some way around it eventually. Like appearing far in the distance, or having one of its Turned to leave me a present. And I'd rather like to have something to fight back before that happens.

Right, onto work.

The Slender Man has the following at his disposal:

• A sense of teleportation
• Some method of sensing a victim beyond human senses
• Moderate shape shifting capabilities
• A series of slave-like individuals to carry out actions that require a constant physical form or human interaction
• A shroud of enigma potentially hiding much detail

The actual process of taking a victim varies. Some are killed in fires started by it, some simply vanish, and others are mauled in grizzly fashions. This implies it can also possess pyrokinetic powers, and inhuman strength, though it is equally likely it is, instead, extremely manipulative of its surroundings, using height to act in place of strength and surrounding materials to produce the flames.

Coupled with this, its weaknesses:

•  For whatever reason, it cannot attack those above a certain altitude. Cause unknown.
• As long as it is under observation, or believes itself to be under observation, it is forced to obey the laws of time and space of this dimension.
• Its assumptions of human nature draw it to, apparently, seek victims in lit areas over dark ones. 

 I hope to God that list is longer than we currently know it to be. The last one is a recent development, as proven by Shaun on Testing 1,2,3. Hopefully I can gain his assistance at some point or another.

Taking these facts into consideration, I can begin to work. Likely Scenario Profiles can be formed. The Imprint Theory has already been posted as my email to M.

Must look into Strahm's Finale scenario. Crime report lists unknown fatality killed by a shotgun belonging to Strahm, opening the potential of Possession potentials. It's entirely possible that The Slender Man is the result of something binding itself to a host, and the corpse found was the result of Strahm firing his shotgun at it. The host died, and Strahm took its place. In which case, victory would require finding a way to fight the binding process.

Enough idle speculation. Research before theorising.