Unrivaled Meganium has arrived, and it seems to be a doozy. It opens up with Reflect and Light Screen before using Curse to take you down. I use a Skeledirge this time around thanks to its hidden ability, Unaware.

Skeledirge @ Shell Bell
Ghost Tera
Ability: Unaware
Modest, 252 Special Attack and Defense
Torch Song
Shadow Ball
Protect

With Unaware, you will ignore Meganium's stat changes. So all you need to do is use Torch Song until you feel you have enough boosts (I did +4) then use a Tera Shadow Ball. When it resets your stats, you should Protect, then repeat the process until you take it down.