After the cutscene on the right theres a glowing A on the cliff side and shoot that with your bow for a bonus and there should be a glowing bow there also. Now go on the middle hill and shoot the archers on the other side, now after another cutscene there will be archers on the boats. Take out the left side first since you need to rescue Trumpkin on the right.
There are fist-sized rocks laying around, pick them up (B) and throw them at the archers, some will be 1 shot kills others 2. After you killed both boats take the grapple and hold LB/RB and swing it out towards the boat and as soon as you pull it ashore you get the achievement.