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