I assume in this case you are actually referring to one of the many Cyclops that you can and will find while wandering around the world. Attacking the helm(or better yet, tricking the cyclops into smacking it off it's own head while trying to get at you as you climb it's body) will allow you access to one of the few weak points these menacing enemies have(it's big bulbous ugly eye, just in case you were still wondering). Damage dealt to these areas is not only greatly increased compared to the other areas of the body, but they often cause debilitating pain to your enemy, forcing them to make mistakes(if not entirely fall to the ground in the throes of wracking agony), and opening more opportunities to you and your companions to lay them low with little or no risk to yourselves.
Finding a new enemies weakness can often times be a challenge(either locating or taking advantage of). Let's take for example another of the larger enemies found within the game, the Gryphon. When first you engage a gryphon(might be spelled griffon, cant remember, apologies) or rather a Gryphon engages you, it will swoop down upon you from high up in the sky, sometimes doing a few strafing runs before it ever decides to land. One of the first things you should notice about this enemy is that it is rather large. It also flies. Being rather large and being quite capable of flight means one important thing. The wings of the gryphon must be powerful indeed to allow it to fly so easily.
So there we have it. A gryphons wings. If you can damage a gryphons wings, whether it is with spell, blade, or bow. You can effectively turn one of your enemies greatest strengths into its greatest weaknesses as well.
Try and keep in mind what about your enemy makes them so strong. Whether its flight, size, reach, speed(yes, it can be a weakness as well as a strength), or any number of other things. If you can analyze new enemies as you come upon them, you can sort out weaknesses and strengths, attack styles and defensive patterns fairly quickly and adapt to each. This is the most important skill to master in the world of Dragon's Dogma, sure you can wing it in every fight you ever get into, or you could become an efficient and deadly adventurer that tears through hundreds of enemies without ever being hit. I'll let you decide on that one.
Hope this helped and good luck in your adventures.
P.S.- Cyclops are also really weak to knee attacks, all that weight can't be supported on a pair of crappy knees. This is usually what I attack first before trying to climb them and get the helm off(if they have one) otherwise just go for the eye.