Running A’s are probably the most practical way of conditioning the hip flexors. But they are more for strength endurance.
As a general rule, there really isn’t much need to specifically focus on the hip flexors regarding strength. They should develop in balance with the rest of the muscles from participation in most strength and explosive exercises. Most of the time, if there is a problem with the hip flexors, it’s excessive tightness (which can of course reduce their power output). I would concentrate first on maintaining optimal length and muscle tone in the hip flexors.