You might change that big flashy "Required" to "Recommended", since not everyone has to use those two pokemon to make it. Players who prefer the experience can pick nearly anything they want, and they only need to know to be above level 80.
Against Lorelei nearly any fast Electric type will do. For a slightly bigger challenge you can use grass or even a fighting type since they are all /ice.
Bruno is too easy. Onix have nothing for spdef, the rest are fighting. If you brought anything with high spatk and psychic moves you're golden.
Agatha is arguably easy as well. All her pokemon are also weak to psychic moves. A pokemon, like Arcanine, that can withstand mega gengar's Shadow Ball(spatk) will wreck it with Crunch.
Lance is where I typically have to let some pokemon fight to the last breath then switch. Gyarados cannot take an electric hit, if you properly EV trained your electric mon.
For the rest of Lance and all of Gary, just enjoy the fight! If your pokemon are high enough and you know the basics it can be done. Queue theme song...