After you capture or KO the legendary pokémon of Hoenn, the balance of nature is restored. You can go into the Sootopolis Gym for the final badge.
In this gym, you will be up against Water-type pokémon, which are weak to Grass and Electric attacks. Don't bring Ground, Rock, or Fire types to this gym, because they are weak to Water attacks.
Cross the Ice
Go north from the entrance and you will find some white tiles of ice. You have to step on each tile once to make the stairs to the next area appear. If you step on a cracked tile, you will fall down below, where you will encounter more trainers. Go south in the underground level to find stairs back to the upper level. After going up the stairs, if you stepped on all the ice tiles in an area, you will be able to walk on them again even if they are cracked, and unsolved ice tile areas will be reset.
In the underground area, you will find these trainers: Beauty Tiffany, Beauty Bridget, Beauty Connie, Lady Brianna, and Beauty Olivia.
On the first area of ice tiles, step on the bottom middle tile, then go all the way left, then go all the way up, then go one tile right, then go down all the way (stopping before the broken tile), then one tile right, then up TWO tiles (not all the way up), then right one tile, then down all the way, then all the way to the right, then all the way up, then go left until the stairs appear. Go up.
In the next tile area, step on the rightmost tile that you can reach, then go all the way left, then all the way up, then one tile right, then all the way down (stopping before the broken tile, then go right until you can go down, then after going down, go right, then go up, and go left when you can, then start zig-zagging to step on all the tiles on the right side, until you have stepped on the tile in the upper-right corner. After that, go left two tiles, then down, then left two tiles, then up one, then right one. The stairs will appear, so go up.
There are two trainers in this area: Poké Fan Marissa and Lass Crissy. Because of the three middle platforms, there are many ways to break all the tiles in this area. As you break the tiles, make sure not to cut off your access to the northern stairs or the three platforms in the middle until the end.
Wallace has a level 44 Seaking, a level 44 Whiscash, a level 46 Milotic, a level 44 Sealeo, and a level 44 Luvdisc. All the pokémon are weak to Grass moves. All but Whiscash are weak to Electric moves. Sealeo is also weak to Fighting and Rock moves.
After you Win
After you defeat Wallace, you get the Rain Badge. It causes all pokémon to obey you.
Wallace also gives you HM05 Waterfall, which you can use out of battle now that you have the Rain Badge. Waterfall allows you to swim up waterfalls. Because of this, you can go east from Route 128 and access Ever Grande City, which leads to Victory Road.