Take into mind that the abuse that hurts the most is the kind that completely removes any sense of control. Have the bully make the bullied child absolutely at his mercy. Tidal Wave mentioned throwing things into a body of water. Perhaps the bully could dangle something important to the child
over a murky lake and the bullied child cannot swim. He'll beg and plead to get his MacGuffin
back, but the bully would taunt him, making him beg repeatedly. This would constitute "getting a rise out of him" which is the ultimate goal of a bully, and in the end, the bully can simply drop the MacGuffin
in the water. Threatening with any such mean spirited prank will work.
Another way to induce helpless is to have the bully insult something that the bullied child cannot change, such as clothes, speech (accent, lisp, etc.), the way he walks, the way he looks, family income, religion.
Have the bully slap, trip or kick the kid just after a teacher walks by so that the kid cannot retaliate without getting in trouble. Have the kid be framed quite often by the bully, it'll drive him crazy and ruin his image. The teacher will never believe him and thusly take away his sense of control in the situation.
Simple pushing also can be terrible. When no authority figure is standing by, it does not make sense to slap, kick or trip the kid. So, the bully and his friends will push the kid between them, being pushed is simple and reduces his ability to retaliate.