This is on the second mission after meeting Yuki, you'll encounter several enemies. Eliminate or sneak past them accordingly, until you come across a guard standing behind an ox by the river bank.
Position yourself above by climbing on the vine to get on the higher ground in front of your objective (Alternatively you can position yourself behind the end of wagon, but as long as your throwing range is within the ox and not get spotted by a civilian, that's also okay)
Use your stone and toss it on the ox and the ox will react and kick the guard and you'll win the trophy.
React fast as the guard on the other-side of the riverbank will witness the incident.