The ideal build for this is 8 sorc lightning dragon or storm sorcery / 2 wizard evocation for sculpt spells / 2 cleric for tempest and max damage on a lightning spell (this can be combined with twinned spell and divinity recharge items).
You want to take 1 Sorcerer level first to get Con saving throw bonus, and make the build int based, using the wizard slots for chromatic orb / shatter / lightning bolt / glyph of warding / chain lightning. Use the Sorcerer spells to take buffs, and with 10 Wisdom you can prep two level 1 cleric spells so can have bless and something else prepped.
For cantrips to scale with Int, take damage cantrips on the wizard level (fire bolt, ray of frost, shocking grasp are all you need). You get guidance / resistance / light from the cleric levels, then burn the sorcerer cantrips on stuff like mage hand / blade ward / friends / whatever else.
Feats wise for MC you only need 1 ASI for 20 INT with Ethels hair, plus Elemental Adept: Lightning. Companions will only get 18 INT.
And then basically you just spam lightning bolt & chain lightning, use chromatic orb & shatter at lower levels.
