I think it'll depend on single or multiple targets.

Single Target:
Eukrasia Dosis and refresh when it's about to run out
Dosis in between everything else (and spam it)
Phelgma whenever you can
Toxicon when you have it up and need to move (if you don't need to move then you're better off with Dosis)

Multiple Target
Eukrasia Dosis and refresh when it's about to run out
Dyscrasia in between everything else (and spam it)
Phelgma whenever you can
Toxicon when you have it up

And to get Toxicon up, in the short time I've been able to get away with playing (I am a work), I've been making it a habit of eating AoE with Eukrasian Diagnosis up, which has been fine for doing FATE so far, the damage is nothing and it's more Toxicons. It makes me wonder how I might purposefully eat AoEs playing Sage if I'm not expect anybody to take damage or whether it'll be a DPS gain or loss.