Don't try to burn down Vahlokzin or the kwama waves too fast. It just won't work. Instead, work the mechanics of the fight, especially if you aren't leveled off the scale.

UNtil the final stage of the fight, Vahlokzin only takes major damage when Sai blows the horn. It drives him up onto the ledge and he lets the kwama do the dirty work for him. The first time around, just light/heavy attack the kwama. They come in two groups from either side of Vahlokzin's ledge; try to kite them into Sai's range so he's useful in the fight either keeping them busy or killing the scribs.

Vahlokzin flies back down after the first kwama wave. Use ranged attacks, and it's not worth burning resources on skills because you can't do much damage. Just light/heavy attack, and stay out the way of any of his attacks and wing sweeps -- just keep moving and whittle down his health little by little. He starts out at 52%, then goes down to 51%. Just before he hits 50%, Sai will blow the horn and he'll take off for the ledge again. All you have to do is harass him until Sai blows the horn -- he can't be killed at this stage.

NOW the fight gets more serious. This is where it's easy to wipe. Use RANGED light/heavy attacks on the kwama again and KEEP MOVING. Don't stop moving, ever. Don't use close-up attacks and skills. Don't abruptly change direction because you'll stop momentarily. Move in wide circles. Don't sprint, either. Just run normally and keep light/heavy attacking the kwama. There are Kwama Guards this time, which take longer to kill and can hurt you more, so keep an eye on your health. Again, drag them into Sai's zone so he also attacks them and does something useful. Vahlokzin will drop fire on you, and one or two hits will wipe you. But if you keep moving, he'll miss. That's why you just keep moving. The fire drops in red-circled rings, and usually from behind you so it's hard to see. The rings are red outlines, not filled-in, so it's harder to spot the AoEs. As tempting as it is, don't rush. Don't burn resources by using skills or ultimate here because the fire will get you every time if you do. Just keep moving, stick to ranged attacks, and take them down bit by bit. Except for the Kwama Guards, they drop quickly enough.

Once the second wave of kwama is done, Vahlokzin will come down for the final round. His health will be a lot lower now, and your attacks are actually useful this time. Don't bother waiting for Sai to blow the horn -- it won't happen. Use ranged or close-up attacks, or a combination now. Do whatever you're comfortable with. Vahlokzin will attack like any other dragon, with tail and wing sweeps, fire and force pushes. Pay attention when he starts to shout and put up your shield; you'll be OK. His "Fus Ro Dah" has the enemy heavy-attack tell of visible yellow lines if his head is in view -- watch for it and shield right away. Since you conserved resources on the trash mobs and minor fights before, now's the time to dump your skills and ultimates on Vahlokzin and get real damage. Pop your favorite potion to buff your health and stats if they run low. At this stage, Vahlokzin is basically like an overgrown delve boss and you can take him down.