Q-Force Legend is a bronze trophy awarded in Ratchet & Clank: All 4 One. It is obtained when 50 enemies are killed using melee attacks in a row, without the player taking any damage.
It is easiest to do this with a human controlled partner, as an AI team-mate will often use weapons to kill all enemies. Having a human team-mate will mean less enemies will attack you, leaving you to use melee on enemies.
This is of course best done on smaller enemies. The Vertigus Cliffs section of N.E.S.T is the best place to complete the trophy thanks to the amount of successive small enemies.
If playing as Qwark, using the Quantum Deflector is useful to not take any hits if it is difficult to avoid enemy attacks.