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).
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Metal Gear Solid:Peace Walker has 33 missions.
1GB
Beat the game.
To get the c96 you must complete extra ops 20 with an s rank
You get the fim 43 you just have to complete main ops 12.