Grumpig also does nothing to affect your opponent's board. All its attack amounts to is "take one prize" while leaving the Night Marcher sat in front of you completely unharmed and ready to KO Grumpig next turn. End result, you both get a prize, but you also lose Grumpig (and by extension the turns you took setting him up) while the Night Marcher loses nothing since the card you KO'd was in the discard pile already anyway. You'd be better off just bringing out something with a half-decent damaging attack and just KO'ing the Night Marcher already in front of you, at least then your opponent might be losing a DCE and a tool card as well.
Bottom line, Grumpig doesn't counter Night March at all. Stick with Trevenant, Jolteon-EX, Giratina-EX or Bats for that.