By changing the team and builds a little bit Im sure you can put another Robin ultimate before 150 av, so this is definitely not the lowest cost possible. But the strategy I use falls apart if you remove anything other than Robin's e2 from this run.
Also, I lost at least 2 attempts because ruan meis lightcone passive just bugged and didn't give her the 10 energy she needs to cast ultimate in wawe 2.