Advance Map is easy to use utility to edit maps and areas in GBA games......
Things u can do through it
1.Edit map
2.Add event or sprite
3.Add Pokemons or edit Pokemon in an area
4. Edit the position of sprites

No since Abhi asked for Pokemon editing I will describe that first

1.Load ROM in advance Map
2.Select By INI files on left list.
3. Select the tab Wild Pokemon(Third from left )
4. Now u know what next!!

ya,
indeed Advance map is the easiest tool for rom hacking