I know this sounds bad , and immoral but , what I did to get pokemon to level 100 quick without directly cheating for one or rare candies was use the code to increase all enemies to level 100 and grind in mount silver
I mean it makes more sense to get the maximum experience for each k/o that you can obtain , especially if you need your team emeber for prime cup in stadium2 (which takes enough attempts to beat without cheating for max stats/using cheap teams full of ubers)
and in g/s you get the beauty of the experience of a lv100 but the stats are still that of the (example of a poke from outside mt silver) lv 44 dodrio still making it no harder to fight them
beware , in crystal your opponents will have lv100 stats aswell as exp
or a lucky egg?
or you could go even thurver and wild modify everything to snorlaxes(they give the most exp)
I got my shark for £4.99 new on amazon for the celebi event in crystal really but I thought of using it for the training speedups seeing I may aswell use it for something seeing I paid for it, rather then it gathering dust
I wont lie I handed out every pokemon I own their nypc event moves so my rapidash can now double kick gravellers to hell and back before they explode on me lol
and before you think low of me for doing this , think , theirs like 120 difrent families/evolution lines andyour expected to catch em all or put em all in the h/o/f in stadium2 to fully complete stadium modes , I mean seriously how is one expected to get the relevant experience to get every one to level 100 (im a completionist so I feel like I have to do this especially where I have added otherwise non obtainable moves/gottena nice pink celebi now)
maybe you should think about it too , an im sure you'd appreciate not having to win millions of battles just to afford the vits to max out stat exp whilst your their for your team ? am I right ? I mean you surely must of , like me played countless hours on g/s/c amongst other games , pokemon themed so surely you deserve (especially in the brutal johto training environment giving little to no exp) a bit of help with that?
also if you want to look at more legit methods , try putting your pokes you wana train in stadium2 and restarting so they get boosted exp , and or if you have the other versions train it their (unless it knows return ofc and you don't have a shark to reset its happieness to max already that is)
if you don't have nother game you can always mod the guy/girls id number so it gets boosted exp , these are what helped me level everything so I could evolve every specie in my current collection on my 6 gbc carts
p.s , anybody from London near Essex , I know a shop that repairs g/s/c batteries for £2 , hope you get their soon (lv 100)
all the best