Hmm... I'd say, use lots of really rare berries like the Pamtre berry (read the tag first, you don't want to waste a berry, do you?). If you can, blend with the Berry Master. You can see him sometimes after you beat the elite four. There are 3 definite possibilities that can happen after you beat the elite four: There is either a Clear-Out Sale at the Lilycove Depot. Store, super-lucky day at the Mauville Game Corner or the Berry Master will appear for a day in the Lilycove contest hall.
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Bike over them really fast. Beware: if you stop, the floor will break.
There really isn't a quick way to get to level 100 fast without cheating. The best way would be to re-battle gym leaders and the elite four, along with wild pokemon. Sorry, you'll just have to be patient.
There is not really a easiest pokemon to evolve...but bug type pokemon evolve really fast
You have to have a mach bike and then you go really fast through all the cracks, but don't slow down. To gain speed fast press B. To turn, press the arrow keys very fast so you dont miss the turn.
If you have the VBA emulator then you must press 1 and do what you want