How to defeat a huge, but surrounded army (on one tile)

So I am playing as Italy and going Italy First, well Bulgaria and I managed to overrun Yugoslavia, and then my ambitions took me to North Africa. I swept across towards Egypt, though Britain and their dominions sent a lot of troops and now I have 31 divisions of their troops surrounded in a single tile - Alexandria. I tried attacking with like 15 divisions but they won't budge. What is the best way to defeat them?

PS I have air superiority