What are the conditions to make a 100% catch?

Solution 1:

Critical Catch has not been sufficiently studied to be understood in Pokémon Go as of yet. The Silph Road (the main "research" group) hasn't determined it yet, largely because it's very hard to study (being rare, and thus highly subject to observational and reporting bias).

In general, it is thought to be unrelated to the ball, berry, etc., and instead is likely simply random. It's possible, though, that it has some other element (such as number of Pokémon registered in the Pokédex, as it does in the main game).

It definitely does work on all Pokémon, including Legendaries and other raid bosses (I've caught a few that way). Early on, when there was a bug that caused the "last ball" to always fail, it was still possible to succeed on a critical catch - I caught one on such a ball, as did others - which gave some slight window into the process (clearly it does "skip" some steps of capture, wherever the last-ball bug was located).