It's not so much a question of what cast, but how do you control the fly when it's got over to the fish through all the unpredictable currents. In truth, it's almost impossible in that situation. I would chance any cast that gets the fly slightly over and quite a bit upstream of the fish. If you get no take, it's unlikely that standing there all day repetitively casting will result in a take.
If it were me, I would try from slightly above the dam/falls on the opposite bank and hang or dibble the flies over the fish.
If you cannot get to the opposite bank and the fish are slightly downstream of the angler on the opposite bank then the angler should move upstream and narrow the angle of the cast to bring the flies over the fish slower and limit the effects of the current.
Failing all that, a straightforward single spey with a big upstream mend in the line.