As you progress in the Fields of Elysium, you will find yourself working through Adonis' quest line.
The mission is
Dead Kings and Pretty Things*** Spoiler - click to reveal ***Once you're in this mission, you'll have to find your grandfather Leonidas.
Alexios/Kassandra will ask for his help and he will tell you to fight him so he can trust and believe you're his daughter's son and she taught how to fight properly.
Once you're done sparring, Leonidas will accept to fight by your side and the achievement will pop.
I've already got this achievement so I'd say it's story-related and can't be missed.