Fight with Valdis has 3 stages.
Stage 1.
1. He takes no damage from your attacks. He uses his sword for massive AoE spells. To get to next stage, you need to hit him 3 - 5 times ( depends on what difficulty you are playing) with those Eyes of Zaramoth that are lined up around the area.
2. When you hit him enough times, he'll fall down. The Azunite appears and takes the shield away. Valdis strikes the shield and his sword breaks. Now, in this stage he will only use powerful magic and melee attacks, but nothing a high level warrior can't handle. Keep attacking him. Note that his health regenerates every time he jumps up and down, so use everything ur party have got and unleash it on him. With time you'll notice that the huge rock formation will be falling apart. You'll need to get Valdis to the very bottom of his health for the Huge Eye of Zaramoth to be revealed.
3. After the Huge Eye is revealed, you'll notice that Valdis can no longer be harmed by any attacks. The only way to finish him off is to hit him directly with the beam of Zaramoth. Best way to do it is to run around and then face the Eye directly so that Valdis is between your party and Eye, then use your Mage's ranged attack to activate the Eye. Don't worry, the Eye is high enough not to hit your party but it is only gonna hit Valdis if done properly. This may take awhile to get Valdis to get hit, so be sure to stock up on potions. Once Valdis is hit with the Beam, enjoy the movie.