I'd suggest using X/Y. Easier games to beat. And you can either use Bank or trade off OR/AS to get your team very near the start of the run. These games are also very generous in terms of level caps for outsider Pokémon.I rarely do run playthroughs with more than 2-3 Pokémon, but I plan them out so they counter each others strengths and weaknesses pretty well. The Elite Fours can be challenging with limited teams (and I don't ever use Legendary Pokémon in my playthroughs), but the levels tend to be much higher since I am using limited Pokémon.
I also came up with a challenge I am going to try on either GSC or HGSS. Defeat the entire game with only Unown. Obviously, you can use your starter to get to the Ruins of Alph, but, once there, fill your team with six Unown and have fun. There stats suck, but Hidden Power has some versatility to it, so it might be possible. For some reason I have always liked Unown (I used Unown G on my original playthrough waaaaaaaaaaaaaaay back in the day. I got him to Lv50 before realizing he woulnd't learn more moves. Oo to be 11 again.)
After easing through Red, I figure I may as well go all out and finish what I've started.I've done a few different themed runs, did a Nuzlocke of Black 2 that I only just managed to beat after Iris's Archeops went ballistic, and I've done a mono. But my current fixation is the Metronome Run. It started out as an attempt to make Y more fun, and I kinda got hooked. First rule is obvious, but I've added a little extra spice for myself:
Since you have to prep the team you use, you also have to deal with level caps, which in some particular scenarios can be rather annoying. I try and buff PP to 16 so I don't have to reload. I've achieved success with Y and Ultra Sun (25 attempts to topple Necrozma), and I'm halfway through an attempt on Red version. My initial attempt used a team that could learn Metronome through level-up or breeding, including a level 5 Mew I scored off the GTS (that one time in Gen 6 when it was so easy to trade for mythicals...) and I've used a fresh team each time, there are actually a lot of Pokémon that learn the move.
- Pokémon are only allowed to use the move Metronome during battle
- Battle style must be on set mode
- No fleeing from any battles
- No healing during battle (with the exception of restoring PP)
- No EXP share
- Gym Leaders/Trial Captains/Kahunas and rival battles must be fought using the same number of Pokémon (evil teams and the Elite Four may be challenged with a full team)
- Held items are allowed