The easiest way to beat the elite 4 is( if you have another leaf green or firered) trade with it and use a strong water type for agatha, lightning for a fighting type ,water for a ground type
lightning or water for a fire type, lightning for a ice type,
and lightning for a dragon type.It is a good idea to have a paty like this
zapdos
Gyarados
Moltres
articuno
Charizard or Venasaur
Joltean
Also these pokemon should be around lv60
and you should take tons of full heals,revives etc.
Good luck
-Mudkip621