[How to progress on charous mountain?]

As the title says, I have a problem with the mountain area at the beginning of EP 18.
 

Spoiler

 

I already figgured out, that in some areas the field can change and my guess is*, that it happens because a Pokémon (either mine or the enemy) uses a specific attack (or there are more, I don't know). As example it becomes snowier, if I let my Mamoswine use Blizzard, but I fail to understand how I can make the opposite happen for example unfreezing the stones in the area with the cave entrance.

 

Atleast Fire Blaze from Rapiddash doesn't seem to work and I have no idea why a less stronger fire move would end up in a different result. So can anyone give me a hint, what I should do? Maybe get some pokémon with sunnyday (or whatever that move is called in english)?

 

*another guess was that it happens randomly after a wild Pokémon-fight, but after some time I don't think thats really the case

 

 

[Ask your E16 question [SPOILERS]]

7 minutes ago, Lightseeker said:

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You should really do all the sidequest becuase it has great rewards sometimes. Like goomy tms etc.

 

But do I have to do them before this episode is finished? Planned to do them afterwards, but if some are becoming inaccessible (because of whatever) at the end...

[Ask your E16 question [SPOILERS]]

Could someone tell me if I should do the sidequests of the new episode before I do the main story? Can I miss something if i decide to do the story first (as example like failing the police-finding sidequest earlier in the game)?