Whenever you attack with one or more Goblins and/or Orcs, create a 2/1 colorless Construct artifact creature token with flying named Ballistic Boulder that's tapped and attacking. Sacrifice that token at end of combat.
Mordor Trebuchet's controller chooses which player, planeswalker, or battle the token is attacking. It doesn't need to be the same player, planeswalker, or battle that any of the Goblins and/or Orcs are attacking.