Would an assault vest Magearna be viable? I'd imagine it would be similar to the specs build, but with the main difference being the change in item. So something like:
Assault Vest
Magearna @ Assault Vest
Ability: Soul-Heart
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Dazzling Gleam
- Aura Sphere
- Ice Beam / Shadow Ball / HP Fire / Volt Switch
- Flash Cannon
Magearna already has excellent SpA, which can snowball anyway with Soul-Heart regardless of the specs boost, and the ability to switch moves is invaluable - I always find it irritating to get the boost then be forced switch, whereas assault vest has more chance to take advantage of the special attack boosts gained by a KO, both by being able to change moves, and as it makes it very able to tank special hits, which gives it far more longevity.
I'd run Ice Beam, because it certainly OHKOs Landorus-I, and Landorus-I only has a tiny chance of OHKOing with Earth power after stealth rocks (before stealth rocks Landorus cannot OHKO, without a crit). Landorus would otherwise force a switch on Magearna, which would allow it to either use a spammable move (knock off or a powerful special attack), or set up rocks.
There aren't many special attackers that could OHKO magearna with an assault vest (Mega-Zard Y's fire blast was the only thing I could come up with, though there are probably others), whereas there are quite a few who could without - Landorus is an obvious one, but I am sure there are others.