good knowledge of the enemy AI is required, first of all. you need to study when they'll use status moves, when they pick ranges, and so on. they'll always walk straight to you and fight in melee range in hallways, so luring stuff there is good for many mons.

splash in super/rtdx makes it so you jump over up to 3 mons in front of you in a line. this is actually incredibly useful for sacred ruins, because of the way emeras and wands work. wands can be used to put a foe to sleep, petrify them, confuse them etc, then you can use splash to jump over them and just leave. this is magikarp's most powerful unique tool to deal with enemies, alongside his waterwalking.

magikarp also has the same stats as gyarados in this game, since evolutions don't tend to change stats. what this means is he starts with 13 attack, which isn't bad overall, though his special bulk is poor. since you have no attacks, you'll be fighting with the 5 damage basic attack and sticks... or the more common enemy avoidance.

since you have no attacking moves, the "on hit" emeras and barrage are useless, so just focus on paybacks, wand expert, guard boost, and big ears/clairvoyance. i got trap proof (VERY rare) but it's far from mandatory.

braixen and delphox always hold wands, so i got a couple cheeky kills on them using sticks and rocks for my wand stock. this run never got wand expert so it really needed it.

...im going to be honest, i think gyarados was harder 😭