This changes the metaphor slightly, and requires a slight adjustment in the phrasing of the sentence, but I think it works and fits the context well in this particular case. A proxy target is an expression used both in artillery and in politics, and the idea in both cases is to aim at one thing in order to hit something else — which is exactly the point of the expression here.

This is from a book about artillery in World War I:

"'Direct laying' takes place when the gunner aims his artillery piece at the target he intends to hit. 'Indirect laying' occurs when the gunner aims the piece at an 'aiming point', a nearby proxy for the real target. This use of a proxy target makes it possible for a gunner to fire at a target that is on the other side of a hill."
https://books.google.es/books?id=0fGcCwAAQBAJ&pg=PA8&lpg=PA8...

Another reference to the same phenomenon here:
https://books.google.es/books?id=EZBECgAAQBAJ&pg=PA63&lpg=PA...

And in the socio-political sphere:

"Sometimes protest can even express its concerns indirectly, through targeting one institution but aiming to affect another—a "proxy target." For instance, French farmers tend to target the government in Paris with demands that actually regard the agricultural policy of the European Union."
https://books.google.es/books?id=PoJTCgAAQBAJ&pg=PA451&lpg=P...

"Sometimes political violence may even express its concerns indirectly, by targeting one institution but aiming to affect another—what social movement scholars call "a proxy target"."
https://books.google.es/books?id=CQmOCwAAQBAJ&pg=PT51&lpg=PT...

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Note added at 16 hrs (2018-12-16 10:49:14 GMT)
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Actually, now that I look carefully, this hardly requires any adjustment: you just have put "as" for "para": "use him as what old artillerymen call 'a proxy target'".