At the beginning The Pass of Caradhras, you'll be given the ability to toss Gimli.
If you stand two large characters relatively close to one another, you can alternate pressing X and Y in sequence.
Pick up Gimli and walk close to a another large character and let the blue target appear and press X to toss the dwarf. Press Y to switch to the character who catches Gimli, line up the blue target again, and press X to throw him back.
Rinse and repeat...