You don't need to put the oven on its highest setting.........
start by putting it in the freezer over night, that will make everything shrink. then use a heat gun or hair drier to heat the outer tube, this will cause it to expand, and the cold battery will still be shrunk from the freezer. If this does not work, then you will need more violent methods.
Is the stuck battery sliding up and down but just wont get past the lip, or is it not moving at all and stuck at the bottom?
Each of those legs holds 3 cells, did the other 2 already come out?
If all else fails, an adequate amount of high explosives can make any problem go away.....