let me get this straight;
you want a monster that can probably kill a mega-beast faster than dwarfs,
Literally is its own class of monster,
can screw a fort up worse than a mega-beast,
has many different abilities,
and killing it with melee will probably kill your own troops.
what is- why would- who could- why would anyone want this? From what you're saying, a single one of them can kill a small group of dwarfs. That is overpowered to the extreme, so what could possibly balance these creeps out? You cant make them weak because the blood would still get on soldiers. you want them to be godlike in melee, so you cant make them weak. Not to mention that the hardest part of this would be to make all the effects and the AI for them.
honestly, while i haven't been here long and I've just been lurking in requests for a few weeks, this is hands down the worst suggestion in the entire thread, there isn't even a basis for the aliens to be in here, you just want them for some reason. If you actually want a challenge so badly don't make traps or anything to block creeps from entering your fortress, or hell, just get a bunch of beasts in cages and release them inside of your fortress, seriously, if you want this so bad you could just mod the game files to make a god monster, just don't come up with with pointless suggestions.