You could do this the Hard way by leveling up one of your most used/favorite characters. Making sure to check off all their personal goals that they need to accomplish to progress to each status all the way up to Legendary.
The Easy way however, is to use your Burn Returner (Also works for Guards) under the Special Teams area. That character has NO personal goals to accomplish. So, just dump Exp into them after each match, they will slowly level and reach Legendary status.
Mine reached that in the 2nd season of Dynasty. I was running 1 minute periods.
This Burn Returner character is a great way to also score a fast touch down at the start of most matches, once they are leveled up. Good luck out there!
You can also do this achievement with traded/purchased characters, you will still need to complete whatever goals they still have at that time.
This achievement can be delayed, it may not pop until you start a new match or new season with that character.