Well, your strategy does seem solid, although I my training for them stopped when Dragonite reached level 68, which was more than enough to survive Salamence's Dragon Claw (if he didn't Crit) and outpace all the rest of his Dragon pokemon. With Ice Beam, Dragonite took care of them all.
Gyarados was no match for my level 64 Electivire, and then Charizard was also taken down by a Charizard level 64, with Rock Slide.
I used the Elite 4-2 themselves as training regiments for my pokemon. I always went as far as I could go without using items and then just repeated. Exp. Share is also a good thing to use.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
