The FIRST STAGE of the battle with Gnashor is quite atypical because he'll be moving underground with great speed. The best thing to do is to target him (SHIFT button) and dodge each time he gets dangerously close to the character.
Wait for the opportunity when Gnashor emerges from underground, standing as seen on the above screen. As soon as that happens, use Death Grip (middle mouse button).
A successful Death Grip attack will throw the boss down, so get to him quickly and start hitting him will all you've got.
After a while, Ghashor should dive underground again, but this time he'll occasionally put a form of blades rolling on the surface (the above screen). Evade the boss by dodging to the sides, and wait for the opportunity to use Death Grip again.
Depending on how much damage you've managed to deal, the boss will go back to moving underground (you'll need to repeat all of the steps described above) or transform into a larger creature (the above screen), which will commence the SECOND STAGE of the battle.
The most vital thing to watch out for during this part of the battle are Gnashor's double whip strikes. It's crucial to dodge them at a precise time. You can evade the whip also by approaching the boss and running around him, given that his size prevents him from moving too quickly. Apart from the whip, try not to get crushed by the boss' legs, though they're fairly slow.
I recommend attacking the boss each time after the whip attack or when he's close to finishing it and you're certain he won't be able to reach you. Similarly to Skeletons, make use of Death Grip to quickly pull yourself to the boss.
Another good idea to taking Gnashor's health points is using the Reaper Form, although for maximum efficiency it's best to use it only after the beginning of the second phase of the battle.
Continue attacking Gnashor until he exposes himself to another Death Grip attack (the above screen). When that happens, quickly press the middle mouse button to force him to change back to his previous form. He'll be defenseless, so run to him and use your best attacks to hurt him even more.
Now you need to be even more careful when it comes to dodging, because the boss will shoot out from underground with large very speed. Dodge instantly the moment you see the first so as not to lose health.
After he changes back to his larger form, you'll proceed to the THIRD STAGE of this battle. This part differs only in that Gnashor will use more sophisticated and less avoidable whip attacks. Consequently, running around him and pulling to him quickly becomes even more crucial.
Continue hitting him until once again he exposes himself to the Death Grip attack. When he goes back to his original form, weaken him, dodge his by now well-known attacks and wait out until you can resume the proper battle (the above screen).
The last FOURTH STAGE of the battle involves another variety of whip attacks, but you should be quite familiar with them by now. Continue the offensive until the boss loses almost all of his health. To win the battle you need to use Death Grip one last time at a precise moment (the above screen). The main reward for defeating Gnashor is the Golden Skull. Afterwards, pick up the items lying around and look inside a chest which will appear on the arena.