I wouldn't hesitate in cutting the whole shrub to the ground is what is called a 'rejuvenation' pruning technique. The sooner the better.

Hollies respond well to this type of pruning, and it will enable your holly to grow back from the ground in an attractive manner. It's recommended that you cut sharply to six inch stumps.

The best time to attend to this sort of pruning is in the early spring BEFORE bud break. Never in the summer or late fall, or even in the winter.

That's what I would do.