How does everyone train their pokemon for these high level battles? Currently have 15 badges so just wondering if grinding through the normal E4 is the best way to level up at this point. I don't want to get the 16th badge and then have no other good way of training.
Training Pokemon for high levels takes a LOT
of time and patience. If you want to level up fast, a good way is to use the Pokewalker all day, to level up a Pokemon while you're away from your game - or fight strong wild Pokemon with your Pickup slave in tow, finding Rare Candies for the rest of your battling team. (If you have more than one Pickup slave, so much the better.)
Also, avoid raising Pokemon with extreme growth curves. (No Hariyama, no Drifblim - nothing that requires 1.64 million EXP points... they take forever and a day to get to lv100.) I prefer to stick to strong Trainer battles - the strongest I can cope with without my Pokemon taking serious damage - and gain experience points that way. Otherwise, if you're training in the wild, it takes ages and ages and ages and ages and ages...
I speak from experience. It took me sooooo looooong
to get my Platinum team up to lv100...