I've went over this in depth in a deleted thread, but Toxikon is entirely useless even in AoE. The damage drop off is so severe that it's actually DPS neutral to just lose a GCD cast. If you have no sting and you need to move, you're actually better off just overwriting your Eukrasia Dosis DoT than you are using Eukrasia Diagnosis for a barrier unless you feel like you need that barrier, which will be pretty rare given how plentiful and powerful Sage's OGCD healing is. Even in harder content, I imagine you'd more likely prefer Eukrasia Prognosis for the group barrier if you really need some healing.

Even in AoE it's useless. Get the pre-pull proc off it, but because of the 50% drop off on secondary targets, the damage is just worse than 2 uses of Dyskrasia. The math on Toxikon is just garbage.