This question is effectively for a coup de grace maneuver that could only be pulled off once per long rest. I'll try to keep what I am trying to do as simple as possible; In this scenario I am a lv 5 sorcerer / lv 2 paladin.

Round one; cast haste on self

Round two; try to hit target with longsword casting green flame blade and expending a lv 3 spell slot for divine smite then follow up with a second attack using my 4th level spell slot for a 2nd divine smite, but with my bonus action stop concentrating on haste and cast as a bonus action thunderous smite landing the hit for a total of 1d8+2+4d8+1d6+1d8+2+5d8+2d6.

For the sake of this question let's assume i have all of my spell slots unused.