They start chucking subtle hints at you after finishing the desert area.
Sakuya was attacked when she was on her way to a picnic, and it stole a Salted Char before running away.
Then Blaise's diner is attacked and the monster steals the Salted Char he was cooking before running off.

To get the bosses to spawn, you have to enter the arena-ish area with a Salted Char and place it in the middle. You only need the fish the first time. If you don't have recipe, it's just a Char (summer fish, catchable at Bone Bridge in desert if it's not summer), cooked on a Frying Pan with no additional ingredients.

There's quite a big jump in stat/gear expectancy between 2nd and 3rd area bosses, so come prepared.

Цитата допису Pickleton:

They start chucking subtle hints at you after finishing the desert area.
Sakuya was attacked when she was on her way to a picnic, and it stole a Salted Char before running away.
Then Blaise's diner is attacked and the monster steals the Salted Char he was cooking before running off.

To get the bosses to spawn, you have to enter the arena-ish area with a Salted Char and place it in the middle. You only need the fish the first time. If you don't have recipe, it's just a Char (summer fish, catchable at Bone Bridge in desert if it's not summer), cooked on a Frying Pan with no additional ingredients.

There's quite a big jump in stat/gear expectancy between 2nd and 3rd area bosses, so come prepared.

I read all of this on a guide before and it was making me wonder why I couldn't get the roar cutscenes to happen and apparently main story cutscenes do not occur if you are on a quest from that message board. I finished the request from that board and then went back to oddward valley and everything started to work fine.