For Reshiram: You have to catch it. If you kill it, it regenerates. Once you catch it, you have the option to add it to your party. Regardless of whether or not you add it, you skip to the N fight.
For level-grinding before the Elite Four: The entrance to route 10. Just keep riding your bicycle back and forth until you see a shaking patch; run in so you find an Audino (if you don't you can kill whatever it is), who gives OUTRAGEOUS high exp. The best part about Litwick is that you can Willowisp Audino and sit back (without having to switch), and then just wait until the burn kills it. With a Lucky Egg held, you'll probably get 5000 exp a pop (or more). Repeat until he's the level you want.
Litwick evolves at level 41 to Lampent, and you can evolve it whenever you want after that to Chandelure, but I'd recommend seeing if you have all the moves you want on it before evolving.