How can you get Pokemon over level 100?

You can't get your Pokemon over level 100. There is one way: using a Game Boy Advance Cheat cartridge - you may migrate Pokemon at any level you like. Simply buy the cheat system/cartridge and get the cheats online or in cheat books, then migrate any Pokemon on any level. 'Migrate' means 'send Pokemon', but you can only migrate the Pokemon you have on your GBA games (such as Emerald, Leaf Green and Fire Red). With a cheat system/cartridge, however, you can have the migrated Pokemon on any level you would like. You could only get Pokemon over level 100 in the first generation Pokemon games (Red and Blue). Through the MissingNo. glitch, if you continued to Surf the side of Cinnabar, you had the chance of encountering a Pokemon (mostly Snorlax) which was over level 100. Unfortunately, if that Pokemon was levelled up, it would go back to being on level 100. This glitch was fixed and therefore does not apply to the later generations of Pokemon games (from Gold and Silver onwards). So these days, it is impossible for a normal Pokemon to be over level 100.