You do not need a fast fire pkmn to beat genesect as he will carry a scarf anyway. Use either Magnezone if you really want to.
Best thing is to always have great typing synergy to take minor damage and take control of the match. Have a good look in OU and pick what you think is a good team. From there keep battling and you will see what works and what does not. You will see weaknesses and you can update the team to close these.
Best advice is to play over a sim like Pokemon Online or Showdown and read the logs after to see how you could have finished faster or where you could have prevented yourself from losing. Stall and Hyper Offence teams are built and play almost similarly. They both take advantage of typing and immuinities to keep pressure up. While this can be said of most teams, the amount of switching done makes Stealth Rock or spikes pretty handy to keep around. Always have a Scarf user or hard hitting pkmn with priority to keep fast sweepers checked. Scizor or Genesect are great at this but share the same counter (barring U-turn or sloppy prediction) Magnezone. While not nessasary for the above reason, a pkmn with taunt or roar/whirlwind is always handy to have around. (Skarmory or Gyarados)
Above all, when building a team, have a win condition that you work to support: Hard hitting set up sweeper whose counters and threats are countered by your own team --- http://i40.tinypic.com/xogxeo.jpg http://gifsoup.com/view7/4072830/karate-ko-o.gif
Mach Punch is not OHKOing Genesect, likely not even with SR and a choice band. While Ice beam will still maim Infernape and likely cause him to get KOed by whatever is the next hit to catch him. --- http://i40.tinypic.com/xogxeo.jpg http://gifsoup.com/view7/4072830/karate-ko-o.gif
Then Infernape will likely faint in the turn he hits mach punch. You are down a revenge killer and his (not a sweeper as I said above) is still there and only wounded. --- http://i40.tinypic.com/xogxeo.jpg http://gifsoup.com/view7/4072830/karate-ko-o.gif
That is irrelevance. I was talking about carrying revenge killers to stop yourself from being swept. You wanted to bring one in against him. He would switch in, get hit, use mach punch and get hit again and faint.
Honestly it sounds like you are almost trolling. --- http://i40.tinypic.com/xogxeo.jpg http://gifsoup.com/view7/4072830/karate-ko-o.gif
Don't worry about it. If you look hard enough in the Diamond board archive, you can see me attempting to make an arguement that HitmonChan, Gyarados and Azumarril all counter Garchomp with ice attacks. --- http://i40.tinypic.com/xogxeo.jpg http://gifsoup.com/view7/4072830/karate-ko-o.gif