Here is the full list of all 15 Ready or Not achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Complete Coyote using only .357 Magnum as a weapon.
Incapacitate a suspect with a C2 charge.
Use the secret command for your SWAT team (kill me command)
Complete Port using only a taser, flashbangs, and bash.
Complete ‘Buy Cheap, Buy Twice', ‘Hide and Seek', 'Ends of the Earth’, and 'Greased Palms’ at C+.
Complete all levels at S.
Complete Ready or Not in Ironman mode.
Complete Ready or Not in Ironman mode without losing a single officer.
Complete ‘Elephant’, ‘Relapse’ and ‘Neon Tomb’ at C+.
Complete ‘Carriers of the Vine’, ‘Sins of the Father’, ‘Ides of March’, and ‘A Lethal Obsession’ at C+.
Complete ‘Thank You, Come Again’, ‘Rust Belt’ and ‘Twisted Nerve’ at C+.
Be killed by a civilian.
Complete ‘The Spider’, ‘Valley of the Dolls’, ‘Sinuous Trail’, and '23 Megabytes a Second' at C+.
Arrest a downed suspect only to find out they were faking it.
Arrest your first suspect or civilian