Seeing how you've got 2 weaknesses to Water and 1 to Flying without that much counter to either, I'd say you could use an Electric type (also since I'd bet the water gym is coming up fairly soon). I think Magnezone would be a solid choice, especially to help out against Fairy types that a few of your Pokemon might struggle a bit with.
As for your Gliscor, you can beat the game with pretty much any Pokemon if you're persistent enough (I had a Zebstrika on my team for pretty much the whole time during my first playthrough) but if you're having a more difficult time than you'd like then I'd say switch it off if you want, maybe for another Ground type that learns Earthquake by level up like Mamoswine, who gives bonus Ice coverage too (though physical Ice moves are a bit hard to come by on it without breeding, I might recommend you keep an eye out for Ice Shard that Swinub learns at 24).