There are 2 ways

Its more of a hint than a secret but there's no hint option so... anyway, to get a decent head start in the game go to any map and catch some lv 5 pokemon (any pokemon, but the higher the Attack stat the better), put 6 on a team and make sure they all know double kick (it goes a little faster if its their top slot so you dont have to change moves) and go to the baby tower. you will fight 8 baby pokemon whose level are equal to the highest level pokemon in your party, which should be 5. They are, in order, Budew, Chingling, Bonsly, Mime Jr., Happiny, Riolu, Munchlax and Mantyke. Keep using double kick and you will eventually beat them all. When you do, you are given one of the pokemon you just faced, or a shinx. the difference is that pokemon is level 125. they are weak at first, but with the right moves they can be amazing. Mantyke with hydro pump in particular stands out. Also, if you get one that you just can't use, keep it until you get 100K and swap it. You will get back a level 125 pokemon (almost any pokemon, including powerhouses like salamence and garchomp)

The other way requires a faction and Sergeant Rank, and a lot of money. Why? because the reward for Sergeants, which cost 1000 points a pop, is a level 150 snorlax! (for team magma and aqua its shiny) so go to your faction, and get one! Now swap it! (or keep it, your choice, keep at least 1, its an awesome pokemon, especially with Recover, since it comes with around 700 HP) You will get a lv 150 pokemon, potentially one of the best in the game. It also helps to sell one of the snorlaxes, especially if its shiny, since at that level it sells for many million (shiny is something like 17 million.)