...I bet they only want "typing for the sake of typing" and do not consider the ramifications of those type combinations, like say, super effective attacks on them. They all have at least four weaknesses!
Fire/Psychic: Victini was pretty good, though that may be due to it's special ability and attacks... Water/Dark: Sharpedo and Crawdaunt are decent, no problem there. Water/Fighting: Keldeo is really good in the metagame, and Poliwrath isn't terrible. Grass/Dark: Shiftry, Cacturne. Sure they aren't the best, but they're not too bad. Grass/Fighting: Breloom and Virizion. These are some of the top grass types in existance.
Steam: http://steamcommunity.com/profiles/76561198023511833 Pokemon White FC: 1420-6147-6059