While I think that immunities are a very useful thing to have, I agree with many others that making the focus of this CAP being immune to various forms of secondary damage isn't necessarily the way to go. Deck Knight's post serves to highlight my primary concerns in this regard. I mean, we can take a look at Fidgit, who can be immune most forms of status (except Burn and Paralysis inflicted by moves other than Thunder Wave) and has Rapid Spin to get rid of hazards and such, but I don't necessarily think that he has served to "stop the secondary" in such a large way.
If we're looking to devalidate secondary forms of damage as a whole, which is how I interpret moi's concept, I would like to propose that we make this Pokemon actually turn the presence of such strategies into a potential liability for the user. We can do this in a variety of ways, most of them based around ability or various moves. My primary idea is that this Pokemon either has an ability that immediately bounces back moves like Leech Seed and Taunt (like auto Magic Coat) or it has a move that does the same, but it would be stronger than Magic Coat in that you can bounce them back at your discretion (turn 1, CAP9 switches into Celebi using Leech Seed, turn 2, the opponent switches in <generic switch-in poke> and CAP9 bounces back the Leech Seed at it). Pair it up with some status immunities and not only does this mean that the Pokemon makes a fantastic switch-in to secondary damaging moves, but it would also serve to "stop the secondary" as a whole, on a metagame level.
Now, obviously what I'm proposing is very powerful, and if people fear that would be too much, there would obviously be many other ways to interpret this/go a similar route. We could make it trade field effects with the opponent, it could only bounce back certain effects, all of this is at our discretion. But I think that taking this general approach, in addition to giving a couple of immunities, is a great way to implement this concept in a way that greatly effects the metagame and the use of secondary damaging moves.