Uhh, well I pretty much use Gliscor + Vaporeon + Heatran on most of my teams, and usually Tyranitar thrown on as well, as they provide a pretty balanced defensive core, and then hopefully the other members of my team can help cover any major threats that those three can't take out. Like some people have said, you really can't cover all DP threats with 6 pokemon, let alone 2 or 3, so I pretty much try to make sure I have my major bases covered and that my team can survive long enough to get its strategy underway