I'll start saying I'm still a noob, I'm reaching 200hrs but my boss experience is laughable. Still, I don't like pvp so I expect them to become my main hobby in pro.
Requirements: are fine I think, a lot of time is needed in order to be able to fight bosses, so I don't think waiting is too bad. For other requirements regarding certain poke/type, I can't say but why not.
Difficulty: already feels unfair, but what bugs me the most is the fact that I have to grind for certain poke I don't even really like, just because attackers are useless. I think we should find a way to keep them hard (they're bosses after all) but not so strangely hard that only tanks can do something. It's just too sad I can't bring poke I like so much as sceptile, lucario or espeon just because they'll get one hit and deal very little dmg. I don't think I'm the only one who feels this way.
Cooldown: as the only boss I defeated is pumpkin, 12 days sure feels long, but with 40 boss, well, it even looks too short. On the other hand a simple idea would be to shorten the cooldown when the boss defeats us, so we can rematch faster.