How to restore health in the Max Lair
The Max Lair is unlike anything else in Pokemon Sword and Shield, as it takes Max Raid battles and turns them into gauntlet-style challenges where you have to beat back-to-back Pokemon without regaining health between them. On top of that, you can only use one Pokemon that belongs to a group of randomly chosen Pokemon that you can choose from which are not even your own. However, there are still ways to get your health back, which will prove to be very important.
After defeating a Pokémon, you have a chance to capture it and then you will have the option to change your current rented Pokémon for that one for the rest of the maze. This will automatically restore their health, so switching Pokémon is a good method, but sometimes you really want to stick with a certain one that's really strong. This is where the other options will come into play.
Technically having your Pokémon knocked out by the opponent and waiting for it to recover in a few turns is an option, but it's not something you can really plan for. Instead, there are a few more reliable options. The first is to energize your Pokémon in battle, which will increase your overall HP and also your current HP.
As you watch the path between battles and select one, you'll want to look and see if you notice a person on the path before or after one of the battles. If you see one that looks like a trainer rather than a researcher, take this path. This will bring you to a backpacker, who will offer you articles. Sometimes they will offer items such as Scraps or a Shell Bell, which can help automatically recover health in battle. Likewise, if you meet the person who is a Seeker, they will allow you to randomly trade Pokemon with them, which will also have full health.
There is another way to restore health, which is the only way to actually restore your existing Pokémon to health without using any items. Keep an eye on the path and look for something in the path that almost looks like mushrooms. This can be found after a battle, so you'll have to fight before you get to it. When you have finished and encountered these items, you will find out that they are berries, which will restore your health and give you a better chance of winning in the future.
Even with the limited healing methods in Max Lair, there are still ways to be successful. Just be careful when playing and you should be okay with your journey to the legendary Pokemon by the end.