So I've been doing my first Adventures and caught my first legendary. This is how it went so far:
1: Lost on the second battle. Worked enough to know what the hearts on the top left mean.
2: Reached the final battle, against Zapdos. Here, I learned that a double KO means the raid boss wins (either that or it uses the same rules as trainer battles). Zapdos KOed an Azumarill with a critical hit Brave Bird, and KOed itself from recoil. Still, we lost.
3: Final battle again, versus Kyurem. A single Hyper Voice KOed three Pokémon. Needless to say, it didn't go well.
Then the fourth attempt went...
The final boss had a Flying-type, and by the third battle we had a Hitmontop, a Duraludon, a Raichu and a Bewear.
Looking for the easy route, I went to challenge a Grovyle. One of the AI partners inexplicably decides to change their Raichu to a Grovyle, and the boss revealed itself to be Ho-oh. And even more inexplicably, Grovyle WORKED. By using Endure every other turn, it didn't get KOed until Ho-oh entered the desperate phase, and we won handidly.
So far I'm loving the experience. It's challenging but fair (at least with AI partners), but it never gets too easy and it makes you feel like you earned the legendary (instead of just throwing a dice and seeing if it's the numbe your wanted).