Tony's mostly been watching out for Enjolras, because Callaghan seems to have taken on a support role while Enj does the actual throwing. So Callaghan's snowball takes him by surprise, and hits him squarely in the face.

He takes cover again, spluttering. This indignity will not go unpunished! "You picked the wrong team, Callaghan!" Tony shouts across the field, conveniently ignoring the fact that Callaghan did not so much pick his team as get chased onto it, by Tony. "We're gonna rain snowy hell down on your sad little igloo!"

He doesn't wait for a target to appear before he starts his next volley: instead, he arcs the snowballs over the opposing team's walls, indiscriminately carpet-bombing the smaller fort. Maybe he can't see where they've hidden themselves, but if he throws enough snowballs, something's got to hit.