Below is a list of all the Mentors in the game and how to make them available.

Mentor	How to unlock
Android #18	Beat the Androids Saga - Level 34 Required.
Beerus (Bills)	Complete Goku's Training - Level 70 Required.
Freeza (Frieza)	Beat the Frieza Saga and complete Ginyu's training - Level 20 Required.
Ginyu	Beat the first half of the Freeza Saga - Level 15 Required.
Gohan & Videl	Beat the Buu Saga - Level 40 Required.
Gotenks	Beat the Buu Saga - Level 40 Required.
Kuririn (Krillin)	Beat the first half of the Saiyan Saga - No Level Requirement.
Mr. Satan (Hercule)	Beat the first half of the Androids Saga (Cell Saga) - No level Requirement.
Perfect Cell	Beat the first half of the Androids Saga (Cell Saga) - Level 30 Required.
Piccolo	Beat the first half of the Saiyan Saga - Level 5 Required.
Son Goku	Beat the Démigra Saga - Level 70 Required.
Vegeta	Beat the Saiyan Saga - Level 8 Required.