Toxikon Phelgma, and Dosis are all GCDs. So, you should do it based on priority procs. You could do a Dosis spam but you miss out on a lot of movement and the extra damage to adds (the first hit of the AOE abilities does not split off too). Plus when Eukrasian barriers break you get a charge that you can use on Toxikon. You get so much Toxikon that you usually don't need to worry about saving them.