If Nellin moves out of the doorway, and both he and Rudolph choose to engage a monster each, then Althion will move so that he has as clear a shot as possible with his bow for next turn.
If one or the other chooses to move and engage a monster, Althion will engage the unengaged monster. His preference would be to engage the Skelly... zombies are icky.
In any case, he will NOT deliberately put himself directly between the two monsters.
If neither of his companions enters the room, or Nellin chooses to stay put and shoot an arrow, Althion will take one step to the right, so he can see down the wide corridor.
* Alternatively, if Nellin doesn't move, Althion will search for traps before moving to the right, if he's allowed to do that "in combat". *
I forgot about Nellin's stealth option. In this case, Althion will hold his nose with his off-hand, and attack the zombie... unless Rudolph has a strong desire to get rotten bits all over his clothes, in which case Althion will gladly hack into the more hygienic skeleton.