Pokémon Choice
Your choice of pokémon will dramatically affect the difficulty of the battle against Groudon. Groudon possesses the inherent ability called ̶0;Drought.̶1; Drought significantly weakens all water-type pokémon and moves used in the battle. This makes squirtle, totodile, mudkip and piplup poor choices as their attacks will have little effect on Groudon. Groudon̵7;s ground-type characteristic does not come into play against other pokémon in this battle, however, so electric pokémon such as pikachu and normal-type or fire-type pokémon are not at a disadvantage. Try grass-type pokémon for the easiest victory. Their strength against ground-type pokémon will quickly overcome Groudon.
Status Effects
Groudon has no inherent weakness against status effects. This means that special attacks, such as pikachu̵7;s paralyzing thunder shock or bulbasaur̵7;s health-draining leech seed, work very well against the monster. Try to keep at least one status effect on the boss at all times. Groudon is a very slow pokémon, and the use of status effects can keep him locked down and unable to battle for the majority of the fight. Your pokémon will often receive additional attacks between rounds due to Groudon̵7;s slowness, allowing you to alternate damaging attacks with those that inflict status effects.
Special Items
Many special items work against Groudon, but the most effective is the sleep seed. The sleep seed allows you to inflict the sleep status effect on the boss. This effect prevents any attacks or other actions during its duration, though it will eventually wear off. Heal seeds and similar items may grant your pokémon an edge if you possess lower levels than Groudon or choose to use pokémon with a weakness to his Drought ability.
Advanced Levels
The most important factor to overcoming Groudon easily is by using high-level monsters. Although a fake Groudon appears in the earlier version of the game, the real Groudon has the statistics of a very high-level pokémon. It also shares all the same abilities and weaknesses as the first version. Leveling your pokémon to their maximum before attempting to enter the Shimmer Cave dungeon will allow you to easily overcome his defenses regardless of your pokémon type or collected special items.