I don't see a rule anywhere that says "noobs" are not allowed to join.
For Bioshock, honestly the only thing I can see that will be challenging is the "1999" stage, which is basically all 4 stages in one go minus the bossfights with an extra "5th" stage and a (Stupidly easy) bossfight in it. Other than that all 4 stages are mostly the same process of "run, defend, repeat, boss, final hold" except for the 3rd stage which has no boss.
As for the bosses:
Stage 1 - Strafe boss and is instakill on contact. Has 3 attacks which are a lunge attack where he lunges towards someone twice (duh), an attack where he drops a green grenade or something which sets you on fire if you get close to it and an attack where he stomps the ground which stops and slows everyone in their tracks for a very short time.
Stage 2 - Instakill on contact if you don't have kevlar, make sure you keep it topped off. He teleports on a random player (undodgeable as far as I know) and will do one of 2 things:
Will have a big red circle around around him, if you don't get out of it in time curtains will drop on the edges of the circle, preventing you from escaping and you will die instantly.
Will do an earth attack thing that goes in a short straight line in the direction he's facing, not instant kill but will kill you VERY quickly if you don't get out of the way or don't have kevlar.
What's interesting is that this boss is (In my opinion) much harder than all the other bosses because he is really trim happy.
Stage 4 - Doesn't move and has 3 attacks.
When the arena is normal he will use his 2 attacks randomly a few times:
A laser that comes from the front, simply jump to dodge it. It's not instakill but it will set you on fire and push you out of the arena into the zombies, if it doesn't kill you along the way.
A small red circle appears under a random player, avoid it otherwise you'll be set on fire.
Then, the arena turns into ice he will use his 3rd attack: A big blue circle will appear under a random player, the boss will then pop up with ice spikes. This is an opportunity to shoot him. Being in the circle when he pops up is instakill. The arena will then go back to normal and the cycle will repeat.
"1999" Stage - Hilariously enough, for a stage that is supposed to be the most difficult of all this is the easiest boss of all.
All he does is pick either the far right or far left and goes from one to the other randomly. Simply stay in the middle and go opposite from where he is while shooting him in the face.