How do you activate the Shockwave 1st/2nd Strike's effect?
From This GameFAQs post How does Shockwave - 1st Strike work?
Press X on the field. Your Servant will attack, and launch a shockwave forward. If it hits an enemy, they'll be stunned. If you start a battle while they're stunned, they will remain stunned for the first action of combat.
Shockwave - Second Strike raises that to the first two actions. Oddly enough, Shockwave - Third Strike does NOT make it into three actions; in fact it takes it back down to one action. So Second Strike is the best of the three.