Idas:
It should be immune to genetic tampering.
However, some little-used chemical combination should be it's weakness.
It shouldn't just ignore stunning, but fast recovery is important.
More importantly, it should never be unable to see things, and have IR-vision by default.
IT is SUPPOSED to be overpowered. The only way to kill it is a group assault and *teamwork*.
If the heads think that they alone can stop it, they should have difficulty. The station should win together or die trying together.
Oh, and busting out of a welded locker is also a good idea for it's abilities.
Basically, it should be resistant or immune to all standard greifing and then some, a powerful foe, and when the station wins, they should feel the accomplishment.
Not some wimpy "kill it in 30 seconds" mid-boss, and not easily disabled. If the players can consistantly restrain it for more than 30 seconds, they are either paranoid, overprepared, metagamers, or it is too weak.
Think about it like this: It must appear nearly unbeatable, and really frighten any experienced player who sees it coming, but neither should it be totally unbeatable. One player with a clever trap should be able to win, but a mob of 40 new players with toolboxes should only barely win, if at all.
You see the works of H. P. Lovecraft?
Do they have some "Oh, we just tasered it constantly until it died 5 minutes later from the constant toolbox smashings" settings? NO! They have a true horror that no sane man would fight against! THAT is what IT should aim at, not some wimpy boss monster that dies to powergaming, not some instadeath nuke of power either. It can't be captured for long, can't be pacified by genetics(unnatural genetic makeup renders standard methods useless, and no alternatives?). You SHOULD be able to flee, on a shuttle or off the map, you SHOULD be able to space it as well. The game mode should not reveal itself, hiding behind a false game type(the original claimed to be traitor), to trick metagamers, though it should use gamemodes where a threat exists so that players are somewhat cautious.
And that is my vision of IT. IT should be a horror story that you usually lose, but can win with excellent preparation and luck. (But IT should be able to defeat such preparations by altering it's play style, making it a constant war of strategy and counter-strategy across rounds)