first get a class with one man army then kill someone on the other team so you can hold X to pick up their weapon then keep pressing Y switching to one man army and pressing B to back out of one man army while pressing X picking up the dropped weapon.

so what your basically going to be doing is pressing X, Y, B rapidly or holding them all down at once. just try to mess around with those 3 buttons and you will eventually get it......be patient takes practice! Keep working at it!

in the end result if done correctly, you will be able to run around and knife people with your one man army package.