Instructions
Play through "Pokemon Mystery Dungeon: Explorers of Sky" until you achieve a Master Rank of two starts, and have then accepted Suicune's challenge.
Select Treacherous Waters from the list of dungeons and you will be instantly transported there.
Avoid grudge traps as you travel through Treacherous Waters. They are revealed after you step on them, and are only triggered if stepped on a second time. Triggering one of these traps will cause all of the Pokemon on the floor to have grudge status, which will drain your Power Points after defeating them.
Avoid battles with the many Pokemon that appear throughout Treacherous Waters' 20 floors by simply walking around them. While winning battles against these Pokemon will gain you experience, it makes completing the dungeon significantly more difficult.
Go to the 20th floor of Treacherous Waters to encounter Suicune.
Defeat Suicune easily with electric- and grass-type moves. Being a water-type Pokemon, Suicune is significantly weaker against these types of moves. Upon defeating Suicune, you will be transported out of the Treacherous Waters dungeon.