FYI, Creeper, as Delila is behind Sarah in her walker, they're not in combat RIGHT at this moment.  Sarah is watching the corridors to the west, ready to shoot any mutants that make it past Arkhah... (who is playing ninja with them at the moment.)  So Delila probably can wait till Sarah opens fire down that corridor before shooting... (as there wouldn't be any mutants in line of sight with her right now.)  Of course, if Delila could "see" a mutant through a wall somehow, that hellish pistol of hers might very well manage to cut a hole through it and hit the mutant behind...