I think this was one of the worst fillers in recent memory. The best bit was Croagunk stepping in calmly and coolly when Piplup couldn't navigate the marsh, and even then that's a pretty small saving grace.
Not even the inclusion of such a character as Professor Oak could save this episode from mundanity. That, of course, is mainly because they've been destroying the professor as a character for a while now, which is my first objection to this episode. The poetry thing may have started as a neat extra facet of Oak's personality, but they've hyped it up to being his only prominent trait now. (And his poems aren't even interesting!) And since when is Oak the sort of man to go wandering off into a marsh when he has an important professional meeting to get to? The Professor Oak we were introduced to when the series began was a wise, punctual, professional man who put up with little nonsense and caused even less. And then the way he solved Tyranitar's problem - merely by looking at its face... that might be believable, though we should have seen more of Oak's skills than that cop-out solution. The worst part was 'merging his heart' with Tyranitar's or whatever it was. Oak is a professor; a scientist! He has been a force for the objective and academic inspection of Pokemon since the first episode (they gave his conference a purely jargonesque title, credit to them, but it was an empty gesture). To be compassionate to Pokemon is one thing, but the show has been spouting off an awful lot of its friendship/hearts/soul-melding/transcendental sort of message in the later seasons, and frankly, we should get something a lot more concrete and scientific from the Pokemon professors, especially from Oak, the foremost international academic of them all.
Croagunk's battle with Politoed: the cliche "slowmo single swipe" scene, and that was it. The proof of Oak's veracity was another cliche: the "thorn in the paw" test with Tyranitar. Another instance of evolution solving a Pokemon's problem, like they did with Gliscor. Team Rocket didn't even have its usual elaborate Pokemon-stealing plot; they just sprang the idea on James in the spur of the moment.
All in all, this was just a very lazy filler. They did nothing to make the episode memorable or worthwhile.