I'm doing a special self-made challenge for Alpha Sapphire. I'm calling it "All The World On One Pokemon".
Basically, my challenge is to pick a Pokemon before the first (actual) gym challenge. This is the only pokemon I get to use against Trainers, barring Double, Triple and Rotation battles (and even in those, he will HAVE to be in the battle, even if he's not the only one in it).
Wild pokemon aren't part of the challenge, so I'm free to swap him out for those. This is so I can complete my in-game dex and not have to try and figure out how to reset to play normally. He also isn't required to be my sole pokemon for things like HMs, so I can have my HM slaves pick up what I don't want/need on him.
At first glance, this doesn't look to be nearly as difficult as, say, a Nuzlocke; but at the same time, not going to be easy by any stretch. I can't switch out my Lotad/Lombre/Ludicolo, which means if he gets confused or something, I can't simply toss into another pokemon to tank while I cure him.
The pokemon I've ended up choosing? Lotad.
It kinda surprised me. I usually use Exp Share to level Lotad into Lombre, toss a water stone on him, then tuck him away into a box. It's terrible in retrospect, I know, but he never fit into my usual team comps.
But he's surprisingly useful for this challenge.
He's resistant to Steel, Ground and Water; he's only weak to Flying, Bug and Poison; and he takes normal damage from everything else.
His natural move pool blows hard, but that's what TMs are there to correct. My plan is to give him Strength (Normal - Atk 80), Energy Ball (Grass - S.Atk 90), Ice Beam (Ice type - S.Atk 90), and Surf (Water type - S.Atk 90) to give him elemental width. With that setup, there is literally NOTHING he cannot do at least 1x damage to.
Although, I'm somewhat tempted to instead keep Nature Power instead putting Strength on him for some semi-randomized elemental damage.