For this fight you will have to get a handgun and ammo.
How to fight mia in the attic.
Once you have the handgun and sufficient ammo you are ready for the fight.
Now head towards the attic for your final fight with your wife.
Luckily by the time you have to face her you have access to the m19 handgun.
Upon arriving there collect the handgun from the table and walk around the attic.
Use fire aid and then move towards the attic and lower the stairs by hitting the red button.
Once you have collected everything you need to head to the attic in order to start the fight.
During the second encounter mia wields a chainsaw which can easily rip your apart if you are not.