DRAGON BALL XENOVERSE

This achievement speaks for itself, but requires something tricky:

First, complete all of the time patrol missions.
Second, complete all of the parallel missions.
Last, use three wishes from Shenron (gathering all seven dragon balls three times) on the "more usable characters" option.

The time patrol and parallel mission steps are relatively simple, but the wishes from Shrenron actually require grinding "Patrollers in training" you may randomly encounter during parallel missions.

The best method I've personally seen for forcing an encounter is the very beginning of parallel mission No.12 "Fierce Battle! Ginyu Force"

From the very beginning of the mission the patroller should be visible; look just ahead of where you spawn along the walking path for a glowing exclamation mark. If a patroller is not standing around in that location, hit start and retry the mission. Also be aware, you cannot retry missions in online lobbies with other players, so this method is only useful in offline parallel quests.

Once they appear, interact with them and fight them. Killing them should net you one material item and a "Key Item" which is one of the dragon balls. Simply finish off the mission afterwards to claim the item. Important note, as you gather more dragon balls they become increasingly rare. You will frequently get only material items from patrollers later on. To break up the monotony you could try missions No. 15 or No. 2 for fun as well.

*Quick edit: In the event a patroller gives you only material items you can retry the mission and still potentially get a key item from what appears to be the exact same patroller; I've personally encountered it and players in comments say the same. Even though they look the same, their loot does change.

In addition to that, it may take several retries for the patroller to appear as you collect more dragon balls. I'm currently writing this solution between loading screens because of this.

Good luck, happy hunting.