I've found each encounter to be totally different.
Sometimes the annie will go full blown nuts on me and provide a very tough challenge.
Other times she will not really do anything remotely dangerous.
I think they all need to be very hostile, attack you immediately upon entry and do their very best to kill you.
Perhaps adding some wingmen into it as well, like the annie's goons. To further add difficulty.
My strategy with the annie's are - Take out drives first. Then just end up spinning around in circles. And can't really fire on you.
Then go for the power plant.
I do agree they need toughening up, but at the same time, they also need to be tough, across the board. One shouldn't be easier than another.