You need 10 rockets, a combat shotgun and around 100 shells.
Step 1: Dig a tunnel 4-5 tiles from the bottom of the map. The first square is conveniently colored white.
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Step 2: Use your shotgun to...
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(Sorry for crappy diagrams - I don't have screenshots handy.)
It looks dangerous, but it's actually pretty safe. Most of the time you won't even take a single hit and if you have a tactical shotgun it's a massacre. As a bonus, the corpses will be stacked on 3-4 squares which is pretty sweet on UV.