I can already tell this is going to be hard to explain but if you test it out you'll find it works.
The 5 stacking buff you get from Steady Shot gives you a free Aimed Shot when you reach 5 stacks. So the trick is to be at 4 stacks, cast Steady Shot, Then cast Aimed Shot *Before the Steady Shot hits*
The idea is that you're casting Aimed Shot while you receive the 5th stack, making it cost no focus, then your Aimed Shot is still procced so you get another one for free, without a cast timer.
And on top of the double free Aimed Shot, you probably have close to 100 focus to use chimera or another aimed even.