everyman322
First Post
First, the Tempest Cleric's Channel Divinity: Destructive Wrath says you can use it "when you roll lightning or thunder damage" and can only be used once per short rest, so it's not entirely clear whether or not you can maximize the damage on both the original target and the twinned spell target, and I'd be really surprised if the DM rules in your favor when you pull this combo out 
Second, if max damage is the goal, why not use Witch Bolt instead? It does more damage than Chromatic Orb at spell levels 5+.
So,
1. Action: Level 8 Witch Bolt(8d12) + Assassinate(if surprised)(8d12) + Elemental Affinity(5)+ Twinned Spell(x2) + Destructive Wrath(max damage)
2. Bonus action: convert Lv2/3 spell slot to sorcery points (BTW, for others who might be wondering, the point of this is to get enough sorc points to twin the next spell and optionally one extra point to also use Empowered Spell on it.)
3. Action Surge: Lv7 Witch Bolt(7d12) + Assassinate(if surprised)(7d12) + Elemental Affinity(5) + Twinned Spell(x2)
= 586 damage (avg) if your DM allows the Destructive Wrath to also apply to the Twinned Spell target, 498 damage (avg) if not, and then Empowered Spell should easily get you to 500 anyway.
Also, the spell limit Raunalyn mentioned earlier only applies when you cast a spell as a bonus action (Crawford said this somewhere, but just look at where the rule is), but yes, according to Crawford, Action Surge would be an exception if that were the case.

Second, if max damage is the goal, why not use Witch Bolt instead? It does more damage than Chromatic Orb at spell levels 5+.
So,
1. Action: Level 8 Witch Bolt(8d12) + Assassinate(if surprised)(8d12) + Elemental Affinity(5)+ Twinned Spell(x2) + Destructive Wrath(max damage)
2. Bonus action: convert Lv2/3 spell slot to sorcery points (BTW, for others who might be wondering, the point of this is to get enough sorc points to twin the next spell and optionally one extra point to also use Empowered Spell on it.)
3. Action Surge: Lv7 Witch Bolt(7d12) + Assassinate(if surprised)(7d12) + Elemental Affinity(5) + Twinned Spell(x2)
= 586 damage (avg) if your DM allows the Destructive Wrath to also apply to the Twinned Spell target, 498 damage (avg) if not, and then Empowered Spell should easily get you to 500 anyway.
Also, the spell limit Raunalyn mentioned earlier only applies when you cast a spell as a bonus action (Crawford said this somewhere, but just look at where the rule is), but yes, according to Crawford, Action Surge would be an exception if that were the case.