How does someone "leave all the beefin' to 50"?
Let's leave all the beefin' to 50 (please)
Em, you're pushin' 50
You can find a video of the song here.
From reading the lyrics in your link, the rapper seems to be talking to Eminem, and name-dropping lots of other rappers. In this context the first "50" is probably the rapper "Fifty Cent"
Hence "Let's leave all the beefin' to 50" would mean:
Let Fifty Cent get angry and complain at people, let's not do that ourselves
In the next line I think "50" is more likely to be an age, so "Em, you're pushing 50" would mean:
Eminem, you're nearly fifty years old
There is a slight chance that it's still talking about Fifty Cent, in which case my only guess is:
Eminem, you're pushing Fifty Cent close to the edge of his anger limit