It definitely isn't, but this set has merits outside of somewhat trapping Heatran. Anything that can or has the potential to remove Heatran from the game AND do something else deserves attention. It still traps Skarmory and a chipped Corviknight well, plus with the increased SpDef bulk it's able to actually switch once or twice into STABs from stuff that it can immediately threaten back (Kyurem, Lele, Tornadus, Zapdos, even Specs Pult only 3HKOs with Shadow Ball), also Body Press is an interesting move for AV sets for the aforementioned reasons. I think AV Mirror Coat is an underexplored setThe problem with that set is that Modest Tran Earth Power has a roll to OHKO. While very rare, they DO exist. It's better to just focus on trapping Tran as much as you can, when you run a set like this -
Magnezone @ Assault Vest
Ability: Magnet Pull
EVs: 248 HP / 148 SpD
IVs: 0 Atk
- Mirror Coat
This set guaranteed lives a EP from Modest Tran after rocks. However, even this set is not guaranteed to work as a 0 SpAtk Tran has a roll to not even do enough with Magma Storm to this set to get reverse KOed from Mirror Coat. On min roll, Max HP Tran only takes 87% back from Mirror Coat. On the plus side, this set does take 2 Lava Plumes from Tran, only seen on 0 SpAtk variants, so you could trap even those rare variants.
I think 87 min not too bad and you have roll to still kill from full however, it is not in your favor at all. You technically can just ignore Modest as they are very rare and then this problem goes away but this set will work against all variants except the even rarer specs versions and eruption variants. At the end of the day Zone is just isn't meant to trap Tran. We can kind of try with this but it there a lot of variants that still won't get trapped by this. It is impossible to umbrella trap all Tran variants with a Zone.