I made sure the rest of the party was well away from the spawn point before I traded. I had Protect V, Shell IV, Reraise, Reprisal, Phalanx, Enlight, Haste, Sentinel and Fealty up when I traded. I used the two Rare items on the monster, in that order, immediately after it spawned and before engaging. I noticed that, in my log, I received no white text saying that I had successfully used either the moss or the root, only the "readies" dialogs in yellow.

Cirrate Christelle used Miasma Mist almost exclusively after that for perhaps the first 30 seconds, after which it was a mix of Miasma Mist and Vampiric Lash.