Legendary Pokémon are meant to be a rare occurrence in the Pokémon universe, but they're everywhere in Pokémon Mystery Dungeon: Rescue Team DX and the player is expected to beat them up.
Legendary Pokémon might be potent threats to everyone without a Master Ball in the regular Pokémon games, but they can easily be dealt with if the player is prepared. It's important that the player enters the battle with the right set-up of items in order to vanquish the Legendary Pokémon in their lairs and become the most feared Rescue Team in the land.
Preparation
One of the key aspects of Pokémon Mystery Dungeon: Rescue Team DX is item management. The only way to increase item storage is by raising the team rank, which is done by completing quests. It helps to quickly get the party to silver or gold rank as soon as possible by performing rescue requests, as having a big toolbox will give you more options in battle and will make it easier to survive until the point where you need to face the boss.
The player will battle many different Legendary Pokémon as part of the story, but they won't have a chance to recruit them until they battle them for the second time. It's best to prepare for these battles by bringing Pokémon with super-effective moves and linking them together.
Items
In most RPGs (especially older JRPGs), status effects were practically useless in battle. This is not the case in Pokémon Mystery Dungeon: Rescue Team DX. Bosses are susceptible to status effects and the player needs to use this to their advantage.
Throughout the course of regular dungeon runs, the player will find lots of status effect seeds (like Sleep Seeds), direct-damage dealing seeds (Blast Seeds), multi-use wands that inflict status effects, buff/debuff effect orbs, and healing items. These should be saved for when the player is going into a boss battle, as they are key to making the battles easy.
Strategy
Most of the boss encounters in Pokémon Mystery Dungeon: Rescue Team DX involves the player's team against a single big Pokémon, usually of the Legendary variety. The player needs to use their superior numbers to their advantage by having their lead Pokémon use sleep effects to knock out the enemy Pokémon and have their teammates inflict lots of damage. Save one or two sleep items for an emergency, as it can be useful to knock out the enemy if you need to take a turn to heal up. Once the sleep items are used up, go for the confusion and paralysis effects, while keeping up the damage with your teammates. Blast Seeds can also do a ton of damage in a pinch. Bosses recover from status effects a lot quicker than regular enemies, but those extra turns will quickly add up. If the enemy Pokémon has an AOE effect, use the time granted by a status effect to flank the boss, by surrounding it. This means that it can only focus one AOE blast at a time.
If the player is fighting a group of enemies, the main tactic should be disabling as many of the members of possible, while focusing on the third. Use confusion, sleep, and paralysis items on two of the group, then focus damage on the third. All you need to do is refresh these effects when they end and pick off the enemy party members one by one.
Pokémon Mystery Dungeon: Rescue Team DX is available now for Nintendo Switch.