This is actually quite close to your idea of a punch below the belt, but without the connotations of a dastardly flouting of the rules. A body blow is a serious setback, and it tends to imply something with decisive and lasting effects, something that permanently undermines the recipient. All this is true of the Spanish expression too. Moreover, and perhaps most importantly, it's a cliché, just as the Spanish phrase is, and a metaphor of combat (though not military, but I don't think that matters). It's a more colloquial and racy expression that you would normally expect to find in such a text. All this makes it, I think, a reasonable equivalent.

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Note added at 3 hrs (2014-08-01 17:43:40 GMT)
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On the other hand...
When you put it in context, you have a preposition problem. In the Spanish, "de uno de los lugares comunes más sólidos" fits both "dislocación" and "línea de flotación": both expressions are naturally followed by "de". But in English "dislocation" (if you use that) requires "of" but "body blow" requires "to", and it's difficult to see how you could draft this part of the sentence so that it works.

So I'd suggest that either you find a way of rendering "verdadera dislocación" that goes with "to", or else use something else for this phrase that goes with "of". It might be

a fatal undermining

or something similar.