I believe the first trainer has ice so I would say a fire type. (Watch out for the move surf, I'm pretty sure she has atleast one that knows it. If your fire type isn't strong enough it will be a one hit KO.)
The second should be either dark/ghost or rock/fighting. For the rock/fighting I would recomend a flying and a water type. For dark/ghost a normal Pokemon that isn't affected by dark type moves but it knows something other than just normal moves.\
Lance is last. He gives a mini speech about how there aren't any short cuts when it comes to dragon Pokemon but that's not really true. Ice Pokemon are effect against dragon types.
By the time you beat the elite 4 your rival will already be champion. I would advise you to get a Pokemon effective against his starter because that will most likely be his strongest. Level up your Pokemon before hand and make sure to by alot of full heals and revives just in case.
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Its best if you have 1 fire Pokemon and 1 ground Pokemon.
The Elite Four is in the Indigo Plateau.
See question "Is this a good team to beat the elite four"'s answer.
well it's probably were there are fire Pokemon or if you have a exp share go to the elite four and keep battiling ;)
you get to level 100 what else