Alrighty! Time to compare some moves!(cheesy, I know)
Psycho Cut vs Zen Headbutt
Psycho Cut has a high crit chance and a base power of 70, which is probably why it has a high crit chance. It also has a 100% Accuracy
Zen Headbutt has a 20% flinch chance(which can be useful), a base power of 80, which is slightly higher than PC. However, it's base accuracy is 90%, which means it could miss when you need it most, like Thundy said
So, my decision is...
You'll need the power to take out really bulky Pokemon like Conk and Azumarill.
So here's the final set:
Trait: Steadfast ---> Inner Focus
EVs: 252 Atk, 252 Spe, 4 Hp
-Knock Off/Ice Punch(those flying types could destroy Gallade)