Light would be a given. I get the feeling that it, Dark, and Psychic would form a triangle of some sort. We all know Dark's immune to and strong against Psychic, Light would likely be immune to and strong against Dark, and Psychic would be immune to and strong against Light. As for one other weakness, I think Poison should be strong against Light types. Not only does Poison need more strengths (and hey, they seemd to make Fighting strong against Dark just for the hell of it), but I'm pretty sure tons of holy, religious figures had been put to death through poisoning in the past (though, I probably should be looking at things from the point of view of a country where the main religions are Shintoism and Buddhism). As for strengths, besides Dark, it should be strong against Ghosts (it'd be considered excorcism. If this happens, though, Sableye better kiss its reputation goodbye), as well as Dragons (I realize Dragons are pretty screwed over in types as it is, and I was just ranting on Poison having few advantages, but this would reference to ancient European tales, holy knights fought dragons a lot. Besides, Dragons are the only type strong against Kingdra, but anything Poison's strong against is already covered by Flying or Fire).