Some Magearnas carry Aura Sphere. Then, your Heatran might cry ;)
0 SpA Magearna Aura Sphere vs. 216 HP / 56 SpD Heatran: 130-154 (34.4 - 40.8%) -- 54.4% chance to 3HKO after Leftovers recovery...unless Magearna proceeds to straight up counterteam and carries something like Aura Sphere or Focus Blast which doesn't hit anything else in particular.
The set itself abuses the fact that Magearna loves to press Volt Switch and Heatran discourages it from doing so in the first place. Also even if it is hit by Aura Sphere due to ct or whatever Jirachi has Wish support and it is not like Magearna will stay in and eat Magma Storm eagerly anyway.