serene grace +air slash or super luck+airslash
Serene Grace with Air Slash because its a chance to Flinch not a critical hit chance.
Serene Grace would increase the flinch chance of Air Slash to 60%. Serene Grace has nothing to do with critical hit.
Super Luck would raise the critical hit ratio of any move by 1 stage. Super Luck is the ability you want for a higher crit. rate.