I'm not sure if you're looking for 5* or 6* or if that part even matters, but I did 6.1.3. It's 3 energy to a 6* OML. I was doing it for the "defeat a 6* with a special" objective and managed to get pts towards 2 objectives at once. I hope that helps, good luck! Oops, I just realized you were asking about the UC objectives, my bad.
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Probably not very energy efficient, though.
Atleast icarus exist for cav objectives , so they suck a bit less