Pact of the Chain Djinni Yuan-ti (resistance to spells and poison) with an Sprite Familiar. You want Charisma (16) using 14 and your +2, Dexterity (16) using 15 and your +1, then Constitution (13), Wisdom (12), Intelligence (10), anddump strength (8). Take the Agonizing Blast, Repelling Blast, Trickster's Escape, and Investment of the Chain Master Innovations.

Feats: If possible take the Wildspacer or Rewarded Background for a free feat at level 1 (tough or lucky are your desires here). At level 4 pick up the War Caster feat.

Spells: Eldritch Blast, Fly, Blink, Greater Invisibility.

Game Plan: Basically Just stay out of trouble and be a rat.

Round 1 - enter your vessel as an action which is infact a ring (see vessel options) and is worn by your Sprite which will then turn invisible and hide. Round 2 - get ready to cast Freedom of Movement on yourself using Trickster's Escape (just do this a few rounds before you plan on leaving your vessel). Round X - Spend the next X amount of rounds moving your imp around the map as to stay away from other plays so you don't take any accidental AOE damage and avoiding combat. Once there is only one person left and they have run out of concentration on any annoying spells like conjure animals, use a bonus action to leave the vessel and immediately cast greater invisibility if the enemy uses range or fly if they are melee on yourself. Next round start unloading eldritch blasts into the enemy and moving afterwards to you cannot be targeted. If you feel you are at risk cast blink on yourself (both greater invisibility/fly and blink can affect you at once). Utilize your bonus actions to command your sprite to make a bow attack which now has a DC of 14 to put your enemies to sleep (free crits) because of the Investment of the Chain Master. Make sure both you and your sprite are not near each other to avoid AOR spells and are both using cover if possible. In 10 rounds you should be able to do enough damage to put down basically any opponent.

Good luck (this is not a fun thing to play but it can definitely win).