Lightning Dragon Sorcerer 13/Assassin Rogue 3/Tempest Cleric 2/Fighter 2
Action: Level 8 Chromatic Orb (lightning) deals 10d8 + 10d8 again (Assassinate's auto-crit and advantage), kick in Channel Divinity for max damage and add +5 (Elemental Affinity), then double it (Twinned Spell). So (20d8 maxed +5)*2 = 330 exact damage.
Bonus Action: Convert a level 2+ slot into 2+ Sorcery points. I suggest a higher slot because you want more points for Empowered Spell rerolls on the next attack...
Action Surge: Level 7 Chromatic Orb (lightning) deals 9d8 + 9d8 again (Assassinate's auto-crit and advantage) and add +5 (Elemental Affinity), then double it (Twinned Spell). So (18d8+5)*2=~172 average damage (higher with Empowered Spell).
330 exact damage + ~172 average damage = ~502 average damage in one round. Happy bursting!
Action: Level 8 Chromatic Orb (lightning) deals 10d8 + 10d8 again (Assassinate's auto-crit and advantage), kick in Channel Divinity for max damage and add +5 (Elemental Affinity), then double it (Twinned Spell). So (20d8 maxed +5)*2 = 330 exact damage.
Bonus Action: Convert a level 2+ slot into 2+ Sorcery points. I suggest a higher slot because you want more points for Empowered Spell rerolls on the next attack...
Action Surge: Level 7 Chromatic Orb (lightning) deals 9d8 + 9d8 again (Assassinate's auto-crit and advantage) and add +5 (Elemental Affinity), then double it (Twinned Spell). So (18d8+5)*2=~172 average damage (higher with Empowered Spell).
330 exact damage + ~172 average damage = ~502 average damage in one round. Happy bursting!