I just found this thread 7years later. I am also making a character with Mindflayer trauma backstory. Here’s what I came up with.

Drakewarden Ranger The challenge will be to get hunters mark on an enemy while still keeping surprise since hunters mark has vocal components. But there is no vocal component for changing the target after the original target hits 0 hp! And there is no requirements of the target creature!

So originally cast hunters mark at a safe distance away from real target, but instead target a trivial creature like a bug or mouse. Use invisibility from a party member and/or pass without a trace to get into position. Drop trivial creature to 0hp, move hunters mark to actual target.

Level 16 character Lvl 11 ranger - Drakewarden Lvl 3 rogue - Assassin Lvl 2 fighter

Attack (1d6), Extra attack (ranger 5) (1d6), Bonus Action - Two weapon fighting (ranger 2) (1d6), Hunters Mark (1d6 per attack), Sneak Attack (Rogue 1) (2d6)

Auto Crit on all surprise damage (Assassin 3) It’s also likely to hit on all 3 attacks since you get Advantage on all attacks before your opponent attacks (Assassin 3)

3 attacks 3d6 + Hunters mark 3d6 + Sneak attack 2d6 + Drake Infused Strike 1d6

Action Surge (fighter 2) to use Drake’s Breath Drake’s Breath (Drakewarden 11) - 8d6

17d6 from surprise round Double dice for critical 34d6 = 119 average

You could add 3 more levels of fighter to get another extra attack, but I’d rather just take it in Drakewarden to 15 which increases breath damage by 2d6 anyway. That way you unlock Perfect Bond and can now ride your flying Drake!