EDIT: Wait, hasn't it been raining in all the footage we've seen of the Wishwashi cutscene? Oh no...the Assimilated Borg Whale is going to devour us all, isn't it?
I also think because of this, it will be the hardest totem to take down. None of the other totems really have the deck stacked in their favor like this.
Like we've assumed Lurantis might be tough but we're operating under the assumption it (1) has solar blade and (2) will summon pokemon that cause sunny day. But that's still a 2 part process that will be easy to dismantle. Statistically it will also probably be easier than Wishiwashi's School form.
And after wishiwashi we have....Salandit 2, Vikavolt, Mimikyu, & Kommo-o
Salandit 2 doesn't have a lot going for it. Corosion will likely still be its sole ability and fire doesn't get a lot of combo potential. I think it will just be like Gumshoos/Raticate: a strong pokemon that you're fighting before it's likely to evolve and summons other, weaker pokemon to wear you down. Since it can poison anything and probably have attract, it will probably try to annoy you
Vikavolt has potential. If it really evolves that late, it probably has good stats (speed & special attack) and it will no doubt summon charjabug to give it an extra boost.
Mimikyu is a petite single staged pokemon with a one free hit built in. It will probably be neglible to take down
Kommo-o has no big gimmick beyond specific move immunities and will probably be easy to take down because it's end game, you're going against 2 pokemon max and you likely have a full high leveld team. Clair's a pain because she had a full team of things to go through, not because it's one really tough pokemon.