Rocket guzzlers

Hi all, this is the second year I've tried to grow rocket. Last year sowed 3 lots and all got eaten. This year rocket germinated nicely. Now outside during day and at night in one of those temporary greenhouses. It's planted in a large pot and I've put coffee grounds all around edge. I thought was doing quite well then two mornings ago I found a slug on the greenhouse wall and snail trails over the pots, so I thought ' I'll fix you lot' and I put saucers of beer slops on every shelf in the greenhouse at night. This morning I found 4 slugs in the greenhouse and half my rocket disappeared. Help- do you think the beer has actually made things worse and what can I do to grow this blessed rocket this year?