Dragon Quest VI: Realms of Revelation/VS. Nokturnus

Intro
At first glance, Nokturnus is the most mechanically complicated enemy in the game, being the only one to sport three attack patterns. 8✮ Liquid Metal Slime + Forbearance takes care of most of Patterns A & B harmless, but Pattern C consists mostly of physical attacks. Dragon Slash & Metal Slash do ~50 Damage even to those with maxed Resilience, Oomph will boost those even higher, and Lullab-Eye can throw a serious wrench into your schemes.

In addition, Goowain will need to cast Kabuff immediately at the start. Otherwise, Raging Roar could kill him Carver, if Nokturnus Stabs him at the start, then goes immediately into Pattern B. Kabuff will ensure that he always survives, and also gives a much needed Defense boost to Rex & Goowain...

...But it also means Disruptive Wave will be used at the start. That sets up a situation where Nokturnus can attack 3 times while transitioning from Pattern B to Pattern C. Keep all this in mind as we go into...

The Strategy
The start is pretty straightforward. Goowain uses Kabuff on Carver before C-C-Cold Breath, then uses Buff on him once Nokturnus hits him with Disruptive Wave.

Ashlynn gets Magic Burst off before Healie tags in, Rex uses Gigaslash as much as he can, and Carver spams Forbearance until the turn after Raging Roar. Healie keeps double-casting Kafrizzle until Nokturnus goes into Pattern C, and Terry uses the Yggdrasil Dew with Hela's Armour when Nokturnus uses Raging Roar for the first time.

After that is when you need to make some hard choices. Carver can't consistently survive Quick Attack AND whichever two attacks Nokturnus decides to start Pattern C with; You'll need to settle for him defending Healie with Selflesness.

From there, your best options vary depending on how lucky you are. Generally speaking, you'll run across one of these three situations...

Carver enters the round with 100+ HP: This is the ideal scenario. Your heroes will keep pounding away while Carver shields them with Forbearance.

Carver enters Pattern C with less than 100 HP: Now Carver could die. Instead of Forbearance, you'll want to just use Selflesness on Healie, have him double-cast Multiheal, and hope Goowain & Rex aren't too unlucky.

Nokturnus uses Oomph on himself: Same situation as Carver entering with less than 100 HP, except now Rex needs to use Disruptive Wave. Healie might also be able to use Kafrizzle, if the party's looking hardy enough.

Nokturnus uses Lullab-Eye on someone other than Carver: If it's only on Goowain, you might be able to get away with just replacing him with another Dragon, as Amos can also use Priest spells. Otherwise, you'll need to have someone use Tingle. Rex & Healie are too important to leave napping.

Nokturnus uses Lullab-Eye on Carver: This is what we're most afraid of. Nokturnus can switch back to Pattern A, and all the devastating AoEs that come with it, at a moment's notice. Healie cannot survive a single C-C-Cold Breath or Kazap, so we NEED Carver awake if we're to survive them...

...So if Ashlynn has the Kerplunk Bracer, now's the time to put her to use. Have her out just in case Pattern A happens while you're using Tingle... otherwise, the results could be devastating.

Nokturnus goes back to Pattern A: This is also an ideal scenario. Goowain & Rex could be hurt if Carver's only shielding Goowain... But Goowain & Rex are pretty tough. They will generally survive, meaning Healie can patch them up on the next turn.

Nokturnus goes back to Pattern B: Now you've already spent Terry's Yggdrasil Dew. Healie must pick up the slack with Multiheal. There's a small chance he'll go before Raging Roar goes off, but, it's a risk that needs to be taken.

Beyond that, the battle comes down to your best judgment. The sections written above are guidelines, not hard & fast rules. You may need to go off-script if Nokturnus gets really crazy with his combos. Amos may need to use Kerplunk to give you a fighting chance. You may even need to use the Sands of Time, if you REALLY get unlucky!

But, you'll generally win. Nokturnus is tough, but, not that tough. If you beat Murdaw & Mortamor, and you've watched the video at the top, you'll usually beat him. You may just need some time to get used to his patterns.