If you're going to try to beat the elite with that team, you need to do some heavy-duty training and TM work. First of all, don't go to war without weaponry. The best type of attack to use against Lance is Ice. If you can get a STABbed Ice Beam off, that's even better, and will likely take down his Gyarados as well. (If not, that's what Ampharos is for, no?) The team you have now can't STAB it, so the best you have is teaching an Ice TM to Feraligatr. If you don't have Ice Beam, why not try the Icy Wind that you got from Pryce?
Will, for you, isn't much of an annoyance since your Ampharos can take out pretty much everything except Exeggutor, out of whom Arcanine makes omelettes.
Koga can be a pain in the neck, but your Espeon will be great here. Forretress will probably be the easiest for you, since you have a representative from each of its weaknesses: Fire, Electric, and Water. Your Pidgeot will be useful for anything that's part bug.
Espeon is probably your best bet against most of Bruno's, since Psychic-types eat Fighting-types for breakfast. However, for that pesky Onix, bring on your Feraligatr and rain on Bruno's parade. Can you Surf? Onix can't.
For Karen... well, she says she's got all Dark types, but she lies. Does your Graveller have Fighting-type attacks? If so, use them on her dark-types and you'll get some super-effective hits in. For Vileplume, you could either use your Pidgeot or your Arcanine. I'd use Arcanine for Gengar, but really, when you get to that point, just use whatever you have that still has HP left if you have to.
Heal up, use your Revives, save the game, and lead off against Lance with Ampharos. Then Ice Lance's Dragonites. You can use either an Ice or a Water attack against Aerodactyl, but make sure that you have a nice water attack to cool down his Charizard.
I do have to ask, though: as far as I'm aware, you couldn't have gotten an Arcanine this early in the game unless you traded. Why not swap your team and raise them up that way? I second everything everyone else has said about training; my favorite training areas are Mountain Road outside of Blackthorn and Victory Road. Your Feraligatr should benefit from all of the rock-types there, and Ampharos would enjoy thundershocking all of those Golbat. When it comes down to it, you have a well-designed team for the Elite Four; it's just a matter of teaching them the right attacks and getting them strong enough to be in a position to use them. Good luck!