Clarification: The memento base glitch is an extention of the egg battle glitch. If you go into a secret base, you can battle with an egg. It shows the pokemon inside (albeit with the wrong colors) at level 5, but more importantly it can gain levels through experience. Here's what to do, roughly:
1) Get a pokemon with more than 10 hp ev's, and over level 34. Empty your pack except for an egg.
2) Go into a battle with a pokemon that can poison- I use my Resttalk Regice against an oddish on route 119, but any other will work.
3) Once you are poisoned, run around until you have one hp left. Use a pomeg berry to decrease hp. The hp should be at 0, but the pokemon still poisoned.
4) Use an antidote/pecha berry. Go into a secret base with your friend. You will be able to battle them. Enter the battle-you will start with your pokemon. Switch to the egg.
The egg will gain experience and grow levels- even evolve when the other pokemon dies. Pokemon lvl. 100 with memento work best because they die quickly. Latios and Gardevoir give the most experience- latios gives more.
The objective is to raise 6 latios's to level 100, delete all moves except memento, and trade records with a friend.
This only works with emerald- if you try this with ruby or sapphire or do it incorrectly in emerald. Then the hp will turn into two ??, and when it engages into battle and receives damage, the hp loops from 65,536 or something like that. It's quite funny to watch. In emerald, you can hatch an evolved pokemon- i.e. a level 5 blazekin. It is the only way for pokemon like agiligross to be used in Lvl. 50 battle frontier play besides for hacking.