21. The Pomeg Glitch
In a nutshell, this allows a Pokemon to enter "negative" HP, which can later be used to train a Pokemon inside an Egg.
You’ll first need a Pokemon at least Lvl 34; the higher the better. It needs to be EV trained in HP; enough so, when fed a Pomeg Berry, its max HP will drop by at least 2.
Once you can meet that condition, drop that Pokemon’s HP down to 1 by whatever means you can think of. Most would get Poisoned, then start walking around with Poison damage until you reach 1 HP, but there are other methods, like Endure. When you’re at 1 HP, clear your status (if any), then feed the Pokemon a Pomeg Berry. It should have ?35/max HP in its HP display.
From here, things get interesting. If you simply want to activate the Pomeg Glitch, then you’re done. The true HP is 65535 when glitched, acting like that number in Water Spout/Eruption damage calculations. Note that Shedinja is not eligible since you can’t increase its HP.
Now, to get an Egg to battle, make sure the only Pokemon you have on your team are the glitched Pokemon and the Egg you wish to battle (to simplify the explanations). Lead with the glitched Pokemon and get into an easy battle, preferably a wild Pokemon battle that you can win with one hit. Win, then switch the Egg and glitched Pokemon’s places and use an HP restoring item (such as a Potion or an Oran Berry) on the glitched Pokemon to make the glitched Pokemon faint. Get into a battle and the Egg will be sent out. Note, that you will whiteout if you don’t use a Revive on the glitched Pokemon before you win the battle with the Egg.
You’ll notice the Egg gains Experience when you win. This Experience is carried over with the Egg, allowing you to level up, level-evolve and even learn level-up moves while in the Egg. This is where a Secret Base full of Lvl 100 Memento Latios will help you gain Experience. When you hatch the Egg, it will hatch at Lvl 5, have no EVs and will keep its form and moves from the Egg.
The other thing this can do is get Missingno. into a battle. Setup a Pokemon to have ?35 HP and enter a battle; switch your Pokemon for another in your party during the fight (the glitched Pokemon doesn’t need to see battle at all), preferably one in the last slot. Except for the glitched Pokemon, deposit all Pokemon/Eggs in the PC. Use an HP restoring item to make the glitched Pokemon faint. Enter a battle and you’ll send Missingono. out; this happens because the game sends out the last slot that a valid Pokemon was used in. Since you no longer have any valid Pokemon to battle with, you send out Missingno. If Missingno. gets hit, you whiteout.