Greetings. If you remember about my Zapdos route to easy victory from before, I am sad to announce that it was removed. Boo.
Luckily, I found an alternate route to OPness that's even faster and easier.
So. You will want to start in Nuvema for this to work. Oshawott will be your starter choice.
Then, get moving. You want to go to Nimbasa City as quickly as possible; as before savescum after every screen.
Once at Nimbasa, go east and maneuver to Undella.
Once at Undella, save, and prepare yourself; this is the first major hurdle.
You want to catch a Golduck on the route south of Undella, and since you can't really hurt it or afford a high-tier ball, you're going to have to brute force it.
Once you do manage to get your duck, you can grab a few more mons if you want, such as a Mienfoo on the same route or a Tangela / Swellow on the route north of Undella.
Either way, get some money. You need 1k.
Once you have it, buy a Net Ball. Then, go back to Undella, save, and surf into the bay.
Your target Pokémon are the Walrein. But there's a catch : their level range is extremely wide, varying from a mediocre 30 or so to a massive 60+.
Try to get one above level 65. Like before, if you saved on the screen your high-level Walrein appeared on, you can reset for infinite tries. That's why we picked up a Net Ball. Note, your duck can have Hypnosis.
Once it's done, grats ! Feel free to Body Slam the game with your grossly overleveled walrus.
There's a bunch of other options for level 40+ mons; off the dome : look around the northeastern part of Kalos (protip : if you want Durant grab it in Terminus Cave), east of Kalos' Victory Road entrance, the eastern half of Unova, Sky Pillar in Hoenn, and pick and choose your team as you want.
Another tip : a strong spread move is very useful vs the villain teams; Altaria east of Kalos' VR entrance pack Dragon Pulse, which is strong, accurate, has a good type, and is a spread move in this game.
Your Walrein will learn Blizzard at level 72 as well.
As far as things to watch out for go, the big one are the villain team leaders. Their Pokémons are really high-leveled, so be careful.
They're not impossible (I've defeated Archie with all my Pokémons on average 10 levels lower than his'), but they do require some finesse.
