i might suggest that you restart the game, but; don't delete the game save data this way you can relive the experience whilst sustaining the team you have, of course if you do want to restart you can simply trade over the pokemon you want to keep onto another gen 4 game if you have one and restart, otherwise do not delete your data, its unnecessary.
as for an unbalanced team, id think that you might not have ev trained your pokemon meaning your most probably better off starting over, or if you want to keep the team member as a team member with evs included you can spend several months growing these berries below.
#21 Pomeg Berry (HP)
#22 Kelpsy Berry (Attack)
#23 Qualot Berry (Defense)
#24 Hondew Berry (Sp. Attack)
#25 Grepa Berry (Sp. Defense)
#26 Tamato Berry (Speed)
if you don't have them then go to the berry master to the leftmost rout of hearthome city, he may give you one if your lucky and the woman with him can sell mulch which can help your farming venture.
easy as answer.