I wouldn’t cross him if I were you, not if you value your life.

【参考译文】

如果我是你，就不会去跟他作对。如果珍惜自己的性命，你最好还是别那样做/ 你还是别那样做的好。

【参考解析】

I wouldn’t cross him if I were you, not if you value your life. =

I wouldn’t cross him if I were you, and you may / might as well / had better not do that  if you value your life.

这里放在虚拟语气后的省略形式not if 是在给出某种建议。