I've been playing Pokemon Ruby edition, when I stumbled across fisherman Dan or whatever and his three useless Magikarps.
I have a level thirteen Mudkip that had fainted, and I was trying to level up my level seven Zigazoon but I ran out of pp for the only attack that does damage. I also have a level seven Silicoon, but it's useless, so I'm stuck in this endless loop with a magikarp that is useless.
Best Answer
It's very nearly impossible to get caught in a never-ending loop, due to the move Struggle, which is the move your Pokemon uses when it can't use any other move.
You can still win this battle, but it will take some planning:
There are a few things that can happen here.
Firstly, if you have Ether or Elixir (or the Max Ether/Elixir), you can restore some of the PP of the attacking move mid-battle, allowing you to continue using your attack. If you don't have Ether or Elixir, you will continue to use up the PP of your non-damaging moves until one of two things happen:
Splash has a maximum 40 PP. Minus the moves you've both already used, if you can survive using your other non-damaging moves long enough, it will be the Magikarp that Struggle's itself into fainting, not you.
Of course, even if your other moves don't have enough PP, you have a trump card. That Silcoon you said was useless? Switch to it. Then switch back. And switch again and so on. Switching counts as a move, and that Magikarp will just keep Splashing away while you don't use any PP. Do this for long enough and the Magikarp will start Struggling, hurting itself more than you, and eventually fainting.
Is it cheesy? Yes. Will it take forever? Yes. But either way, someone's gonna have to Struggle themselves into fainting, and it may as well not be you.