From the newest PF2 preview:
The fighter can use the special certain strike action, which lets him strike with the following failure effect.
Failure: Your attack deals the minimum damage. (Treat this as though you had rolled a 1 on every die.)
So with certain strike, a failed attack roll isn't actually a miss—your fighter is so skilled that you still get a glancing blow on a failure and miss entirely only on a critical failure!