I don't think that Einschlag is suitable - it suggests bombs or meteorites, which, though they might land with a wallop, wouldn't normally be considered as such.

Schlag is fine as explanation, imo, but Watschen, or the other regional words for it, would be even better ,since wallop is a colloquial or slang term, and therefore ill-matched to Schlag

Just found in a message today:

United States: Rainstorm "wallops" New York City area, forces transit delays.
NEW YORK (AP) Torrential rain flooded subways and rail lines and delayed flights early Wednesday at New York's three major airports and thousands of commuters were unable to get to work.

So "Einschlag" does not really match here.
Habe aber auch keinen besseren Vorschlag als "Watschen". "Schwer treffen, beuteln, heimsuchen, ...