Pokémon Super Mystery Dungeon/Chapter 6

A Junior Expedition Society Member Is Born!
So as you can probably guess by the title of this section, yes, you do become a Society member, and that adds a ton of new features that you have to deal with. Nothing that serious though.

The chapter starts with a cutscene of an Ampharos arriving at Serene Village. He's standing on the tree with the hill saying that this is the place, and that he found his way there even if his sense of direction is... less than ideal. Huh. Should be interesting.

Waking Up
You wake up to Nuzleaf complimenting your scarf, saying it looks "mighty fine" on you. He says he wants one himself (not that he gets one). Exit the house, and head to the village square. A cutscene should play of Ampharos bumping into literally everything, before hitting the wall of the Cafe and sitting down. After a cutscene in which he introduces himself (and drops this really cool orb thing), go to the entrance to the school path to initiate another cutscene in which your partner realizes that they skipped class yesterday, and that they will definitely get in trouble if they go to school today. Well, there's only one way to handle this. Suck it up, and deal with it. I mean, what's the worst that Watchog can do?

Finding Ampharos
After a quick cutscene in which Espurr helps you get to school (without trouble) by pranking Watchog, class will begin. Save and Continue.

After school, head over to where Pancham is and watch how he reacts to your scarves. If you don't care, head to the plaza (for more cutscenes)! After a cutscene where your partner realizes that he's seen something like the orb at Kecleon's shop, head over there and ask him about it. After yet another cutscene in which Raticate tells you that they saw Ampharos near the gate, restock your seeds (if you forgot or didn't care), and head on over to the gate. After one more cutscene, head through the gate, to Poliwhirl River!

Poliwhirl River
Poliwhirl River, just like what Carracosta said, is a dumpster fire. There's a ton of Electric and Water types, so if you don't have a Grass type (or Pikachu), then you're gonna have a problem beating this dungeon. Lotad can be encountered here, so be prepared, because he is a Water/Grass type, so Electric will work on him, but Grass won't. Psychic types like Drowzee and Sproink can also be found here, so Riolu is not the prime choice for this dungeon, if you chose him at the beginning of the game. You can unlock Pelipper Island if you die in this dungeon, which lets you use 3 starters to rescue yourself when you die in a dungeon. Useful!

The final floor (Floor 9) is always the same (because, yet again, another mission that requires the floor to always be the same), and therefore, should be easy to find the exit again if you end up dying in the dungeon (and you can't use Pelipper Island).

BOSS: Poliwhirl Brothers (Poliwhirl×3, Poliwag×1)
This boss is extremely easy if you have Blast Seeds and/or Pikachu. First thing you need to do when the battle starts is to use a Slumber Orb. If you don't have a Slumber Orb, skip this step. If you do have one, use it, and attack the middle one first. Use an alliance to weaken him, then finish him with a super effective attack (or Blast Seed). Then focus on the next one. Poliwag only has around 50 HP, so you don't have to kill it right away if you don't want to. The Poliwhirl Brothers have around 100+ HP (estimated), so again, should be easy if you have Pikachu.