Okay, first as a suggestion for your facing the Gengar, go with a dark or steel type if you want to counter act the ghost and psychic moves which sound the most dangerous. If you are willing to train a Shiftry or Cacturne you will eliminate all effective hits from Gengar. (Dark Grass type not weak to grass, electric, ghost, or psychic)
Second, if you really want to use your torterra, be sure to have a dark move, such as crunch, and if you want to protect it against flying type moves try stone edge, your torterra can learn it and it does great damage.
Finally Gengar is weak to dark, psychic, and ghost, not ground because it does have levitate. Gengar also has massive Sp. Atk. but low defenses so if you can survive a hit and have a decent pokemon with a powerful super effective move you should be able to take it out.
And is there anything I left out?
Also marter had a point with the weavile, and tehweave had the dark and steel types right.