you can beat her 1st poke with whatever way then use lightning rod ability seaking to sweep the rest
seaking, ability: Lightning Rod, nature: modest
iv requirement: only require spatk iv 26+, the rest is doesn't matter
moveset: surf, ice beam, rain dance, agility
holding item: mystic water (can be bought in johto azalea town shop for 2k)
her 2nd poke will use thunder punch to your seaking
but lightning rod seaking is immune to electric move
and seaking's spatk would raise for 1 stage
keep using agility to wait for spatk raising 6 stages
then use rain dance when the spatk is max
sweep with surf unless the opponent is weak to ice move
ps. you can use electric move to test the goldeen whether it's lightning rod or not
it helps saving $$ for pokeballs
lightning rod seaking is good to beat some other bosses too