AF Stigro 20 
Awarded for defeating AF Stigro
you get this achievement after completing missions- rescue GA transport,attack on pa-n51. you then have to complete defeat af stigro.
AF Eclipse 20 Awarded for defeating AF Eclipse
you can get this achievement by completing rescue ga transport then complete 2 other missions to unlock defend megalis which when completed you unlock this achievement.
AF Cabracan 20 Awarded for defeating AF Cabracan
you fight af cabracan at the end of chapter 1 you must complete the following to unlock it as the final mission attack on naval port of mimir, ambush sphere invasion forces and attack on richland.
AF The Spilit of Motherwill 20 Awarded for defeating AF The Spilit of Motherwill
as far as im aware you can never miss going against motherwill at the end of chapter 1 so complete defeat spirit of motherwill and you should get it.
AF Great Wall 20 Awarded for defeating AF Great Wall
to get to great wall you must complete defeat unknown next on chapter 2 then you unlock defeat af greatwall then you should unlock this achievement.
AF Jet 20 Awarded for defeating AF Jet
to get to af jet you must follow the league path and go against orca later in the guide i will explain this more. for now once you follow the leagues path you will unlock the mission dfeat orcas special af unit which is af jet but unlike others there is 3 of them. once this missions complete your achievement will unlock.