pokemon-fire-red-leaf-green
I've made it to the Pokemon League in FireRed version with my team at level 45 and intend to grind roughly ten levels before going in.
Of course doing so on Victory Road would seem the first stop though I am apparently not allowed to use the VS Seeker in there. Would travelling to another area be more efficient for gaining Exp by doing rematches with trainers?
Fire Red and Leaf Green will use the Generation 3 EV/IV formulas as that is the generation they were currently in when they released the game.
As for EV/IV training in general, check out this guide: Big Training Guide - EVs, IVs and Natures
Since Generation III, effort points have been completely separate values from base stats. Defeated Pokémon give out 1, 2 or 3 effort points to a particular stat, depending on species (see list of Pokémon by effort value yield).
However, in battles that do not give any experience (such as in the Battle Tower or if the Pokémon is level 100, although the latter is no longer an issue in later generations), Pokémon will not gain any effort points. At level 100, a Pokémon's stats will be one stat point higher in a specific stat for every four effort points gained in that stat.
Pokémon are limited to a total of 255 effort points per stat, and 510 effort points in total. However, since stats are calculated by dividing effort by 4 and disregarding the remainder, only 252 effort points are required to maximize a stat. In Generation VI, the limit was changed from 255 to 252, preventing wasting EVs.
Vitamins add 10 effort points, but cannot raise a stat above 100, or raise the total above 510. These Vitamins can be found for 9,800 in Department Stores or in Laverre City in Generation VI.
For more information on EVs and IVs check out Bulbapedia or simply Google Pokemon Effort Values or Pokemon Individual Values and I'm sure you'll find the information and guides you're looking for.
You will need the National Dex to evolve Shuppet.
Depending on the randomizer you use, you should have had an option to enable the national dex. The randomizer may include a tooltip on why that is important. In the Universal Pokemon Randomizer, this is found under the Misc Tweaks tab.
This is supported by other users reporting being unable to evolve pokemon in the National Dex in threads on reddit or yahoo.
Best Answer
Once my Pokémon were at Level 55-ish, I used the VS. Seeker on One Island, on Kindle Road, specifically against Crush Kin Mik and Kia, as well as Black Belts Hugh and Shea. They are right next to the hot spring room (Hugh and Shea are walking back and forth in front of it), allowing you to heal your Pokémon when needed. They give about 1000-1600 EXP per rematch. [source]
Another good place is on Cycling Road. Koffing give a good amount of EXP as well and they are plentiful there. However, there are plenty of Pokémon that know Explosion, so you might want to keep on your toes. [source]
Other options are available, as well. Some might not be available till after the E4, though (specifically those in Four Island and beyond).