10% or less I did on Shipyard Infilitration. I finished the level normally, then near the end, before you jump on the ship, I found an ammo supply crate and stood by it and kept emptying clips into the air. I did this for about 15 min to get it below 10%.
I then finished the level and the last part is just hand to hand.
For over 90%, I hear you can do the same levels over again. So try the Professor levels twice.