My suggestion is a lot bridges and anti-flexion/anti-extension movements (like swiss ball pikes, roll outs, etc.). If you do any crunch, you need to set your pelvis in a posterior tilt. The posterior tilt is best described as the midnight meat train. Stand up, squeeze your glutes, notice how your pelvis tilts. Lay on your stomach and do the same thing. Midnight meat train. Lay on your back and do it again. This position, as long as you hold it throughout a crunch, will reduce hip flexor involvement dramatically. You'll also notice the ROM will decrease and the exercise will become more difficult.