This can easily be done in the very first mission as it does not have to be 100 unique enemies.
You will be introduced to the blinding controls and for this achievement, you just need to hold
Once you get to the large stairs before you approach Talion, turn around and you will see a patrolling group of orcs near a stationary group, totaling about 12 orcs.
Get their attention and blind them. Keep holding
and it will continue to count for each one that is repeatedly blinded. Blindness is time-limited, so they will recover.
If you don't get it here, you can use the same method on any large group you find. There used to be a video of me doing this, but it has vanished. If you need it, comment and I will see if I can find it.