Crit Fishing.
Faerie Fire + Prone + Greater Invisibility + Oath of Vengance + Reckless attack feature would result in a very high crit chance. I'm not sure if it's enough to break anything but that's where my concern would lie.
Variant Half-Drow Blade pact Hexblade 5 (Thirsting blade, Eldritch smite, Devils sight), Vengeance Paladin 3 , Assassin 3, Sorcerer (any) 3, Battlemaster 3, Gloomstalker Ranger 3.
ASI: Elven accuracy.
Pre combat, you're hiding in the darkness (invisible to all creatures with darkvision thanks to Gloomstalker) with your pet Owl (obtained via the
Animal friendship spell as a ranger). With Expertise in Stealth (assassin), you're darn hard to notice (+17 to Stealth checks).
Approach your mark. Initiate combat, initiative is rolled.
On your turn cast a silent spell (Sorcerer - metamagic)
Faerie fire or
Hold Person with your action from hiding (so as not to break Stealth), and then use your bonus action to place an Oath of enmity (Vengeance paladin) on the sucker. Direct Owl to provide the Help action.
Same turn, move to target. Action surge. Take the attack action. Swing twice.
You're now attacking a surprised (+A thanks to assassin) creature lit up with a Faerie fire or paralysed (+A) from hiding (+A) while invisible (+A), while your Owl familiar trolls it (+A) and it's under the effects of your Oath (+A), and you get to roll an extra dice thanks to Elven accuracy (+A).
Thats 8 x d20 rolls; pick the highest.
On a hit, it's an auto-crit (Assasin, and maybe also from being Held via Hold Person). Spam a superiority dice, a 3rd level slot for a Smite, and a 3rd level Warlock slot for an eldritch smite (also knocking him prone).
8d8 force from the Eldritch smite. 8d8 radiant from the normal smite. 4d6 from the weapon. 2d8 from your Sup dice.
You lose 2 'advantages' after your first attack (the Owl and Hiding) but is now prone thanks to Eldritch smite, so you get one back.
Attack again.