You're not told this beforehand, and are encouraged to use Electric-types, but Altaria, being a Dragon/Flying-type, only takes normal damage from Electric attacks, and it knows Earthquake, which OHKOs most Electric-types.
Even my non-Electric-types, like my Kadabra, my Lombre and my Marshtomp, got totally wrecked by it. I basically just save scummed and kept spamming my Pelipepper's Steel Wing, and when it inevitably got KO'd, I sacrificed my other Pokemon to Revive it.