Some observations:
In my experience, no student's union has ever had good beer. Cheap beer? Yes, but good beer? No.
"Don't piss with the beer!" - this is not a thing English speakers say. 'Don't ****1 with the beer' makes sense. 'Don't mess with the beer' makes sense. 'Don't piss in the beer' makes sense, but changes the meaning (although some SU beers may be improved by this action).
But 'Don't piss with the beer!' is nonsensical and wierd.
1a perfectly good anglo-saxon word starting with an 'f', for clarification
ETA: I just noticed that you wrote 'p!ss', not 'piss', which makes the phrase even more meaningless. 'P!ss' is not a word, but it is somewhat pr!ssy.