Night Slash and Shadow Claw are 2 moves that have 70 Base Power, 100 Accuracy, and high critical hit ratio of 1/8. The only difference, the types.
Your moves (with super-effective):
1) Iron Head: Ice, Rock, and Fairy
2) Poison Tail: Grass (and Fairy)
3) Dragon Claw: Dragon*
These moves only cover 5/18 of the types. Now look at Ghost and Dark:
Ghost) Psychic and Ghost, with dark weakness, and normal immunity
Dark) Psychic and Ghost, with fighting, dark, and fairy weakness
Both provide the same type coverage. Normally, I would choose Night Slash, but in this case, I would lean slightly more to Shadow Claw. The reason is because it has more neutral effective to cover those other types. Now you're super-effective against 7/18 types.
But, I should highly recommend you getting a fighting move like revenge/superpower. The reason being is that your other 3 moves are weak against steel, where fighting isn't. Also, ghost is immune to normal, but fighting is super effective against it. If you take this suggestion, I think you should take out poison tail, since it's not serving much purpose. Now, you're super-effective against 9/18 types. So, if you want, go for shadow claw.
Your Choice! Hope I helped! :)