How do you catch 6 level 78 level Pokemon in Pokemon platinum so that you can beat the elite 4?

Answer 1

Level 78 Pokemon are extremely rare, in fact, it probably is impossible for a wild Pokemon to even get that high. The only way then is to train your Pokemon. When training, try to get a diverse range of types and moves so your team won't be weak to a few moves like having an all weak against fire team or all flying team.