If something is easy to pick (collect, gather), it isn't a challenge, so it means that the other person is going to be easy to beat, defeat, etc. I think it should be 'easy pickings'- the apostrophe is probably there to show pronunciation.
I agree with TDOL. "Easy pickin's" is an old expression, so it probably was used to show how easy something was to harvest or pick from a tree. The apostrophe is often used when quoting spoken English to show how the speaker pronounces (or in this case, does not pronounce) the word. The original expression was "easy pickings." Have a good night.
You gonna be easy pickin's 
I thought there is something called "easy pickin"... and you gonna be its/his
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