Dragon Quest VI: Realms of Revelation/VS. Nokturnus

Note: This Boss Fight is one of the hardest in the game, so a link has been provided to demonstrate it:

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, Healie will need to double-cast Kabuff at the start. This way, we can trigger Disruptive Wave in Pattern B (so Raging Roar & Quick Attack aren't on the same turn), and give the others a much needed Resilience boost.

The Strategy
The start is pretty straightforward. The most important points are that Carver uses Forbearance, while Healie double-casts Kabuff. Healie will then double-cast Kabuff again, once Nokturnus uses Disruptive Wave. Otherwise, he'll double-cast Kafrizzle in the meanwhile.

Goowain keeps using C-C-Cold Breath (or Fullheal on Carver, if he's hit before Kabuff), Ashlynn tags in for Rex (when convenient) to use Magic Burst, Rex uses Gigaslash, 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 Selflessness.

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 is likely to die. Instead of Forbearance, you'll want to just use Selflessness on Healie, while Healie double-casts Multiheal.

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 use Kafrizzle, if Carver doesn't need healing.

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. Otherwise, you'll need to have someone use Tingle, as 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 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. Have her out just in case Pattern A happens, and use 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 Healie... But Goowain & Rex are pretty tough. They will generally survive, meaning Healie can just patch them up.

Nokturnus goes back to Pattern B: Now you've already spent Terry's Yggdrasil Dew. Healie must pick up the slack with Multiheal.

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 random, but not actually that tough; not with our preparations. If you beat Murdaw & Mortamor, and you've watched the video at the top, you'll get him. You may just need time to get used to him.