I liked it because you could figure it out methodically, and while the moves used weren't everyday (boring) competitive moves, you used two competitive pokemon.
It was quite straightforward I guess, but still enjoyable to solve. Firstly you knew that Pokemon A is faster than Gengar (and isn't scarfed, as it changes moves the next turn). You also know that it naturally resists Fighting, so that leaves you with Starmie, Azelf, Alakazam, Skymin, Mewtwo, Crobat, Deoxys, and Ninjask. Now move Y must be a move that changes the user's type, since none of these both resist Fighting and are immune to Ghost (in fact nothing is unless gamefreak makes a Ghost/Normal type). Conversion and Conversion 2 were my first thoughts, but they are unique to the Porygon family and Smeargle, who can't outspeed Gengar without a boosting item. Then I remembered Camouflage, and it was quite easy from there.
Like I've already said, it was well thought out and fun :).
EDIT: At below poster - I think it's a bit too long and has too many variables, and perhaps not enough information (I wasn't really taking anything in while reading it, but that's probably just me). Try shortening it a bit and having only three or four mystery things, it'll make more people want to have a try at it.
