At least they get large minuses for trying to hit an invisible target. In some campaigns you could now trick them into setting off their own traps trying to catch you. 
No alcoves, but the room is only 70’ across. It is round again. Open double door opposite where you entered, but about 45° south of the main passage.
In the south of the room, directly in front of one of the partial archer barriers, there is an open pit. You only got a little more than 10’ into the room, so you could easily make a mad dash back out. This would greatly minimize the arrows coming at you.
My advice to you is to take a hint and beat (hairy) feet, Mulligan. 
10’ in, grief I did do badly didn’t I. 
Exits stage left, through the doors, in a hail of arrows.
The party sees or hear arrows and a bolt come flying out the open doors.
A short time later a surprisingly healthy Mulligan makes it back to the party. (that extra -4 to hit is big)
Gil, can we kill those orcs now? My axe is thirsty.
Give the party a run down of what I saw in the chamber in terms of white powder, barricade, rope traps, Orcs etc.
Anyone have a spell which cause gusts of wind. Having the white powder blown back over them would be very funny, not to mention inconvenient for them.
I have no spell like that, but I’ve got a strong inclination to proceed (behind the fighters) to this room and clear the orcs.
“Come, my friends, come!” Thoroncir says. "Let us not tarry, but press on to glory! Elendil!" Bearing Mulligan’s warnings in mind, he draws his longsword, raises his shield and charges.
Resists urge to outstretch one invisible leg in Thoroncir’s way, Renee would never forgive me 
Hey, don’t worry about her - I’m the guy with the longsword!
Ensure you leave clear lines of fire when you enter. Targets in priority order: Orcs on unarmored or injured companions, spellcasters, archers
Forewarned about the rope traps, Gil-Gandel heartily approves of at least a circumspect charge, figuring that he at least is good for five times his weight in orcs in an archery duel.
You’re doing it again, Thoroncir
So the party follows the somewhat reckless Thoroncir down the passage. Gwaelur is happy to keep up and Theogrim steps it up. Gil-Gandel’s warnings of rope traps seems to have the correct affect as the front three move in and cautiously watch out for traps.
There are 6 orcs in the room, two have bows and fire immediately, one has a heavy cross bow and is trying to reload.
The arrows fly at Thoroncir as the first one to enter. They both miss and one goes whizzing past, just missing Renee.
I bet you say that to all the girls
I’ll keep an eye out for anyone caught by the rope traps or other problems and free them.
Considering you are light enough to walk atop snowdrifts, what is that, one or two orcs? I can beat that!
Indeed, Gil-Gandel fires at an archer hits despite the cover. His orc dies. Brin shoots the orc try to load a bolt and he hits and it dies.
Mulligan happily makes use of all the flour being tossed up by the front three and starts cutting the ropes. Gwaelur chops on in half and Theogrim steps on one.
Thoroncir avoids tripping on the lines and gets an orc wearing only leather. He hits and it dies.
Gil-Gandel fires at the other archer. He hits the barrier instead of the orc.
Brin drops one charging the front with a spear. It skids through the flour and drops.
Gwaelur exclaims, “Hey that one was mine!”
The remaining orc with a bow fires at Thoroncir again and hits him for 5.
Gil-Gandel fires at him while he is exposed and puts the arrow through his neck. He gurgles a bit before collasping to the ground.
The other orc uses the confusion and dust to dive through the hole in the floor. A loud thud is heard.
Few questions.
- Am I still invisible?
- Is that a hole in the floor or a trap door that needs opening?
- Does Thoroncir likes getting injured because it increases his chance of Renee laying on hands?
- I think you are.
- I’ll bet it is. Let’s go!
- No comment.
Oh, were you asking What Exit?
Ghân, cautiously following behind the Fighters + Archers remarks to Renee (who is still singing?), that Ghân should do the small Cures, because Renee’s Cures are more powerful.