Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
On the first one, Heatran will counter most of them pretty well. As for rain teams, Quagsire or Gastrodon are quite good options for counters, as they will not take any damage from Water type attacks.
Most of my losses are either because of my misplays or because of the ever annoying hax. Only a few are because the team, so I guess it's pretty solid.
Darkrai is and must be legal in Ubers. Is cause the guy who told you probably plays on Wi-Fi a lot and has definitely no competitive knowledge whatsoever. Cause if he did, he'd know that ALL pokemon are allowed in Ubers. Heck, the only one that could possibly go beyond Ubers would be Arceus. And finally, must, cause if not, DL13 would be in a ragestorm.