The problem with Aron is that Hail does damage, as does Burn, Pain Split, Leech Seed, Nightmare, Curse, etc.
You know, Clefable learns Magic Coat in Gen. 4. So it has another advantage over Aron. It can bounce back Stealth Rock, Taunt, Encore, Spike, Toxic Spike, Whirlwind, Roar, etc.
Clefable also gets Recycle, so if you Leech Seed a foe, and then switch to Clefable safely through a stat up/non-damaging move, then it can use Recycle to stall with Leech Seed healing it. (You don't want to use Endeavor with Leech Seed because it will then kill the foe, but only heal you 1 HP, so it is best to just Recycle as Leech Seed does the damage) repeatedly.
CLEFABLE LV 2 @ Focus Sash
Magic Guard
~ Endeavor
~ Encore -Hits Ghosts who think they can set-up-
~ Recycle
~ Magic Coat