They can be found around destroyed bastion.
They are weak against Ogroid Oil, Axii & Quen.
Cyclopses can easily be recognized by the single eye located in the center of their foreheads. If for some reason that is not visible, other tell-tale signs are their enormous size, incredible strength and a seething hatred for all humans.
Cyclopses have meaty health bars and plenty of range on their attacks. The biggest problem when fighting them is the fact that it's very difficult to correctly time your dodge rolls so that you don't get hit as you're rolling. As such, quen is essential during these fights, as it'll allow you to take at least one direct hit.
The best way to do good damage to these giants is to only move in for an attack after they've finished their own. Cycloses make use of rampaging charge attack that can be avoided with relative ease, and it presents the perfect opportunity to get behind the monsters and get one or two slashes in. Rinse and repeat, for the most part.
Alternatively, you can use the axii sign, which can stagger the beasts and leave them vulnerable - but it won't do so for long.