Family Fighter is a new battle pet achievements which were added in the game after 7.3 updated. It similar to the previous achievement Family Familiar. Agus added 18 kinds of corrupt pets, so as the Family Familiar, each time, it needs each syngeneic pets of the team to beat once when you beat with all the ten series pets, then you could get the achievement and a pet from the achievement: Felclaw Marsuul.
These corrupt pets have a leader buff (damage reduced by 50%), and it is legendary quality.
If you want to get this achievement, you need to know very well about pet battles and have plenty of full-class pets. Different pets have different battle strategies and you need to match different pets to beat them.
Compared to the Family Familiar, Family Fighter is much more simple. Family Familiar needs to beat different trainers. But the other conditions are all same with Family Fighter -- needs each syngeneic pets of the team to beat once. The point is these trainers belong to world quest, they refresh randomly.You just could meet 1-3 ones each day. You will need several months to do this achievement if you are unlucky. But Family Fighter is different, these magic pets are always there. You could challenge them anytime when you are ready to get the achievement.  
Are you ready? Go and Try NOW!