Two more red monsters, one of them lying in wait to surprise you.

You can’t get up the stairs while that monster blocks the way, and you can’t harm the monsters by shooting them.  But the red monsters can be killed if they are shot by their own kind.

Shoot the monster on the left so that it clears the path for you.  Then walk up the winding stairway.  (The remaining monster will take a shot at you but it’s too far away to hit as long as you keep moving.)