Personally what I expect out of a Cloudkill attempt is likely two spells:
-Probably a less extensible and physically smaller version of Burning Shadows, but as a mist has a vastly higher surface area, it'd inflict nastier burns, especially to things which are normally 'shaded' by equipment, and while less affected by light conditions, replace it with a care for wind conditions. Eyeballing it, probably Fiendish difficulty without the staff, and a valuable addition - Ulgu doesn't have a lot of pre-Battle Magic offense after all.
-A Battle Magic version of the above, scaled up. More massive, less vulnerable to wind, probably needs to exploit some shenanigans to make it big enough, maybe doing silly buggers with shadow conducts spell -> spell creates mist -> mist casts shadow -> shadow conducts spell, of the like, since Mathilde's more mechanical/elemental paradigm doesn't do very well with stretching metaphors beyond reason.