用心

In my opinion, 尽一份力 is 帮忙 in chinese. 帮忙=help, give a hand, lend a hand, do a favour 你能尽一分力证明他是说谎的=You can help to prove that he was lying.

I think "尽一份力" is close to "做出一些贡献", so I would recommend "make contribution to...". "尽一份力" is different from "尽力".

You can do something to prove that he's lying.