How do you get Xfim-54 in Metal Gear Solid peace walker

There is no such thing as the XFIM-54 in Metal Gear Solid: Peacewalker, nor is there in any other Metal Gear Solid game.

If you're talking about the XFIM-92A, then you need to complete the C-Difficulty ops in Outer Ops (not C+, just C).