This card either doesn't work or works realy weirdly and unintuitively. You choose targets as you cast the spell, but you don't get the experience counter until the spell resolves, so limiting the nubmer of targets based on experience counters on a card that changes the number of experience counters is a bad idea.

"Creatures with power X or less"?

"You get an experience counter. When you do, up to X target creatures can't block this turn..."

That sets up a trigger delay so you can target correctly.

That said, the power level on this card looks weak to me. Compare to Panic, Stun and Panic Attack. Keep in mind that all three of those cards are commons.