Try color curves instead of levels. Color curves allows much more precise control over limited tonal ranges whereas levels pretty much skews the entire range.

To adjust only the faces you can create a mask. Cookie cutter might be able to handle this if you can work with a rectangular shape. Put the video on two tracks, one right below the other. Apply the cookie cutter effect to the upper event with a long narrow rectangle covering the face area. This punches a hole through the the video to expose the track below. Adjust the brightness on the lower event and only that part of the image will show through the upper event and be brightened. Apply some feathering to the cookie cutter to make the effect smoother so it doesn't look like a hard rectangle of light.

If the rectangle doesn't cut it you can create a mask in any photo editor, white everywhere except the faces and black where the faces are, feathered of course. Insert this on a 3rd track above the other two and apply the mask generator effect. This will work the same as the cookie cutter but let you use any arbitrary shape you wish.