These holly bushes were 18" tall ten years ago. In the corner to the left out-of-sight is a 20' tall holly tree. I have to trim everything about every 3 months. The wall behind the hedge is 7' high, so I try to stay level with the height of the wall.
This is all you need, a Black & Decker trimmer.
If you get any branches that are too thick, just get one of these lopper shears.