[Danger Mouse]

Ok, ok, dumb mistake on my part...Rotom-H is not good against rain. Okay, first of all, you shouldn't be running life orb on Rotom-H...choice scarf is its most useful set. Also, the idea isn't that you necessarily straight-up OHKO Genesect, but that you force it out without it doing anything useful, and if you predict the switch-in well, you might be able to get a decent chunk of damage off on whatever comes in. Of course, you need to keep rocks off of your side of the field or this strategy won't work at all. Also, getting rocks + one layer of spikes for Deo-D isn't THAT hard, it just depends on what the opponent leads with...Genesect and Tyranitar will probably just keep you to getting rocks up, but not always.

EDIT: To above poster, yes Heatran is better against genesect, and a better pokemon in general, but it has a big glaring weakness called dugtrio that Rotom-H doesn't have to worry about. I was just suggesting Rotom-H as a solution to people who find heatran's dugtrio weakness unappealing. Also, the HP Ground thing is true as well, but Genesect running HP Ground is kind of bad...sure you beat Genesect's greatest enemy, but you lose out on a valuable coverage move in the process when you're just better off u-turning away from tran.