This trophy is obtainable during Chapter 20 - Caravan. You can unlock it either during the story campaign, or afterward by using Chapter Select.
Shortly into the chapter, you will be on horseback and chasing a convoy of trucks. What you'll need to do is ride your horse up next to (or behind) a truck and jump from your horse to the truck. Do not just pull the enemy off the truck while hanging from the side. Instead, you need to climb into the truck and defeat him with melee attacks. Now jump back on your horse and ride to the next truck, and repeat the process. If your horse is not there right away, just wait a few second and he will come galloping up to get you. There are more than enough trucks to get this trophy, so don't worry if you miss one or two. Also, do not try to ride up to the two truck with the mounted turrets. You need to kill those with RPGs located on another truck.
Note: The trophy will unlock as soon as you get the 10th melee kill.