Defeat enemies with the vehicle, long gun, melee, and grenade, in that order, in 20 seconds.
Sounds crazy, right? Well, it can be done at the end of chapter 3 during the ruins encounter.
As soon as you start, there will be 2 enemies directly ahead of you, so run one of them down with your jeep. Then jump out and kill the other with a long gun, vault up towards the tower and melee the next enemy, then throw a grenade up into the tower to take out the 4th guy.
You can restart the encounter as many times as you like until you get it.