There are more practical combos out there than shard + tesla, but creating lightning weaknesses does open up more possibilities for monster hunting.
Against single monsters, so, for instance, mana storm will deal 19ŚMA raw damage to each affected monster.
In no case does the formula refer to a total damage, so don't worry about there being a damage penalty for multiple targets.
The formulae do not indicate damage to secondary targets for splash from R/MR or summon boulder, though.
Temp inactive again...
GigaBowser, 4411 HP melee dark/fire/lightning wizard, 193/204
Deudalephon, 4411 3:1::LK:WT melee water/lightning soul master, 150/165
Hemathion, 4411 3:5:4::WT:HP:MA melee dark lord, 140/140
Anchors, 1414 MA priest, 244/250
Vadise, 1414 MA wind/earth soul master, 210/216
IreneAdler, 1414 MA fire/lightning soul master, 154/152
So, like, let's go look for the road and talk about our hair!