Well, a couple of photos would have helped, as the feasibility of the following solution depends on exactly where the dent is, but here goes...
Unless you have a very strong trained mouse with a small sledgehammer to get inside the case and hammer the dent OUT from INSIDE, the basic method is to hammer the surroundings DOWN from the OUTSIDE. I appreciate that this may not appear to be what one might call intuitive, but it usually works.
You need a piece of rectangular bar material that can be forced into the dented case, so that the affected face is bulged outwards. You then need an adequate brass or bronze hammer (such as those made for use in situations with an explosion hazard) to hammer the surroundings of the dent flat. This reduces the depth of the dent. if the dent is a deep one, you will not remove it in one session, but may have to pack up the flat bar again so that the side is bulged out again, but with a now somewhat reduced dent. Hammer the surroundings flat again and repeat if necessary until the dent is removed or at least so reduced that the case is usable. I am lucky enough to have a pair of bronze wedges to adjust the thickness of the bar assembly.
I have used a similar method on round items, such as a Frenchhorn, the barrel tube on a German
Gew. 88 and the magazine tube of a Winchester LAR. In the particular case of an Enfield magazine you will need to adjust the internal flat bar assembly and the external support on which you lay the magazine to be hammered so that the pressed ribs on the sides are not flattened!
Takes time, but it works!