Finally beat T12 Mephisto. Immortal Sorc made it easy.
Edit: added a planner
Easy T12 Mephisto kill with Sorc's Elemental Attunement aspect (lucky hit: chance to reduce defensive skill cooldown), Endurant Faith charm, and Ice Armor enchantment
The rest is the same as regular Ball Lightning sorc. I used Mekuna's set up on Mobalytics: https://mobalytics.gg/diablo-4/builds/sorcerer-ball-lightning?ws-ngf5-1=activeVariantId%2C1
The idea is to reset Flame Shield with Elemental Attunement so that we are almost always immortal. If unlucky, Endurant Faith and Ice Armor enchantment can help.
Forgot to show Flame Shield skill tree: take duration bonus, cooldown reduction and firebird (cause meteor rains when it ends).
My gears are not that great, and I made some mistakes in the fights too but still managed to beat him pretty easily in T12.