The boss:

• She can spawn on a clean island. No rust needed.
  
• She will spawn after our characters are non-sleep for 24hrs in game, 24mins in real life.
  
• Sleeping at a campfire outside reset the spawning timer.
  
• However, once she has been spawned, sleep at a campfire does not de-spawn her.

Misc:

• Red for blast
  
• Green for impact
  
• Make sure to recite those two colors correctly.
  
• Every resource is capped at 99. So be sure to cook/transform them into food/potion rather than stack them up too much.

I'd suggest maxing out relationship first before confronting the boss. So you'll have max HP and combat power. Also choose a map with campfire and clean surrounding neighbor maps (no rust/monsters) -- prevent unnecessary HP loss by running into them.

When you're ready to tackle Beruberu. Go back to the nest and

• Make 5 healing balm and 5 speed tonic.
  
• No need to make omni-thing, but having them when there's spare resources are ok.
  
• Prepare some top-tier foods.

Eat. Sleep. And when you wake up, grab some food with you (2 basic food as a round-trip ticket for our sky-bus, at least 1 best food for picnic later at the campfire) and go out.

Go to a map with camping area. Stay near the bridge to other map. AFK for ~22 minutes IRL. Then change map to trigger game save. (Optional: go back to the main screen and bump up save file if you wish.)

Now, just wasting time around in game. The characters might complain about their hunger. You might hear a roar and feel earthquakes. Just keep wasting time for about 1 minute. If she's not spawn yet, take a bridge to other map (prefer the non-campfire one) and continue wasting time.

When she does spawn (it will happen when you change into a new map and so she will fly out from the bridge that you've taken), luckily that she's so slow, fly away from her in a way that she's not block the bridge. (Optional: back to main screen to manipulate save files.)

Avoid her and fly back to your camping area. Eat. Sleep. Wake up and go back to the map she spawned. She'll waiting for an epic fight there. However this fight will be a lot easier than other guide since your stomachs are full!

In general, apply tonic to increase charge rate. Don't forgot to re-apply it every now and then.

There are 3 phases of the boss fight.

First phase: DUO burst first, DUO impact second. Rinse and repeat.

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Second phase is observing boss's aura:

• Red for DUO blazeburst
  
• Green for DUO impact

Don't aim for full charge. Just the basic one is ok. The most important thing is trying to get the spinning wheel to land on the right spot.

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Third phase is spamming burst one after another (avoid using DUO burst since it's take too long to charge), mainly to get the boss in the stunt-lock. And when she charge up a terrifying laser attack, shield up both characters.