The consensus seems to be that the poison-fire combination makes the Tier 3 mission too hard. I tried it twice and had to use 1000 CE revives both times, whereas I don't recall ever hitting that level while testing Legion of Almire.
Perhaps the designers could add a sprinkler system to some rooms? One of the Rescue Camp missions has a sprinkler system, so this could work in a similar way. It could be activated by a switch and temporarily extinguish the oil fires in the room. The sprinkler would stay active for a few seconds before the switch toggled off again.
EDIT: I just completed Tier 2, and it was much easier. I had to use only one CE revive for the whole mission.
+1