Pokémon Ranger/Grimer Outbreak

Back at Fall City's Base
Joel promotes you to Ranger Rank 3 for doing an excellent job with your mission! With this, you can use the Poison Poké Assist, the Psychic Poké Assist, and lastly, you'll unlock a second bar to your partner's energy gauge, meaning you can now use a more powerful assist with it.

Just then, a civil engineer runs in pushing you to the side, as he reports that the sewers, which are supposed to be clean, are full of sludge! It's infested with Grimer. Joel assigns you your next mission, to go and capture the Grimer and then deliver them to an expert Ranger that can deal with them.

Grimer Outbreak
Your next mission has started, so head out of the Ranger Base and go the same way you went to the lab (just north of the Base). Go past the lab, and south to find the sewer entrance. The sewer has a good few new Pokémon to capture, starting with Totodile.

Pokémon in this area (Bold = New):

Before you can go on and capture anything else, you'll find some puddles of sludge, and you'll find your first Grimer. After capturing it, the sludge will be cleared, and the Ranger that was at the entrance will come running to take the Grimer with him.

Heading east and south to the next screen, you'll find a few more Pokémon, including Corphish which we hadn't seen before. Notice the two metal fence gates, and how you are yet unable to clear them.

Heading south and east, you'll find another pair of Grimer. Capture them and continue heading east, you'll find more new Pokémon: Drowzee, Koffing and Raticate. The first two are not necessary unless you wish to fill the Browser, however you must capture a Raticate. After you do, bring it back to where you first saw Corphish, and use Raticate's Field Move to cut down one of the metal fences. Head north to the next screen. Along the way, you'll be jumped by a new Pokémon, Croconaw.

Keep heading east to find another pair of Grimer, capture them and continue moving along to the next screen. You'll find two pipes, you can go into the on the left but it'll only allow you to catch a Drowzee and a Corphish, so head into the pipe on the right. You'll meet with Aria, who will go on ahead with her investigations. Go north out of the pipe, and down the stairs directly to the right. Dodge all the Koffing and make sure not to fall down, and climb the stairs on the right side of the room.

Head down the pipe on the left, save if you wish and head west and north to the next screen. Crossing the bridge and walking a bit west, you will encounter a Grimer trio. Capture them and head west to see Aria and a small post on the other side of a waterway. You'll need to find a way to cross, so go north to find Tangela, a new Pokémon.

Bring this Tangela down to the place you need to cross and use its Field Move on the post to get across. Before going any further west, go north and capture a Meditite.

After capturing the Meditite, jump down onto the side you were previously and go east to find a breakable wall. Use Meditite's Field Move to clear it, and go inside to find not one, but two new Pokémon, Magneton and Oddish! Keep in mind that Magneton's Field Move is an improved version of that of Magnemite and Pichu, so it may be a valuable ally to you in the future.

Cross once again using Tangela's Field Move in the area with the Meditite and walk into the pipe to the west. Follow its linear path until you exit. In this room, you can go to the west and find a gate that requires Raticate's Field Move to clear. Thankfully, going down the pipe to the left and straight down, ignoring the branching path to the left, will bring you to an area with another Raticate, so capture it and bring it back to that gate to clear it. If you do, you'll find another new Pokémon behind it, Crawdaunt!

After capturing it, head back into the left pipe, and this time go to the left branching path and then down, out of the pipe. Capture another trio of Grimer and head on through the area. You'll see a civil engineer getting attacked by two Grimer and a Muk. Go to him and capture all three of them in your next boss battle!

This battle is made quite difficult by the fact that both Grimer and Muk leave poisonous tracks on the ground as they move, which hurt if you draw a line into them. To give you an easier time on this fight, you can capture the Drowzee that appears a bit before this battle and use its Assist, as the Psychic Assist levitates the Pokémon in the air for a little while allowing for a much easier capture. Otherwise you can also attempt to use your partner's Assist to paralyze them all, but if you do make sure they are together as it may be difficult to draw loops around them if they are not.

Once you've captured all three, it'll be...

Mission clear!

Aria will meet up with you, advising that you should go report that you've cleared out all the sludge from the waterways, so you both head out, as a Go-Rock grunt watches on as their plans fail yet again.