DRAGON BALL XENOVERSE 2

After you finish all the training missions with Krillin, a message will occasionally pop up after completing missions that says "Krillin is offering a special class in the Business District". When that message pops up you can find him on the southwest part of the map next to the accessory shop with a yellow diamond above his head. His training session consists of delivering milk to people.

The fastest way I found to get the class to pop up is to just do the initiation test for Yamcha repeatedly. I was able to finish that mission in about 5-10 seconds a piece. I had to complete it around 10-30 times before the message would pop up again. I noticed I only got Krillin's special class whenever i did Yamcha's missions instead of both popping up.

If you still need the achievement for Yamcha's training sessions as well I would recommend doing the Raditz initiation test instead for them both to have a chance to pop up.

Edit: Thanks to Philip Andrews for pointing out that all 10 training sessions must be done on one character to earn this achievement

I'm not sure if everything im about to write is a factor but here i go.

First off I found krillin by the school and completed his 4 training missions. Then I went about playing the game.

Every now and again I would get a notification on the top right of the screen saying krillin has special training for me along with where I can find him.

Go to set location and interact with him. This will trigger the missions needed for the achievement. It's a fun throw back to the original dragon ball where you have to deliver milk.

I didn't see a timer so to avoid stupid mistakes don't rush, use your map and you have to return to krillin to complete the mission.

Repeat.