Is it better to do it individually (A carry + the one i want to grind) or should i use 1 carry and 4 champs?

I currently have drexthar kimi dreng arbiter i want to put mastery. I heard that, if u grind it, you can  savev 160gem per champ. Which is quite a lot. 

Takes around 2100 energy and 150 runs per champ.

But does it makes a difference in efficiency if i do all at the same time? Or should i do it individually?

Disconsider the possibility of lvl up the champ in minotaur. Mine are at lv 60 already.