Ok off the top of my head.

Assuming a surprise round and top initiative Dex 20 (becaus of fighter bonus feat) .

Sharp Shooter, crossbow expert max damag rolled always crits.

2 attacks deal 64 (average 60) Dread ambusher - 8 or 40 depending on how you read it (average 33) 2 more deal 64 - action surge (average 60) 1 bonus attack deals (crossbow expert) 32 (average 27) Sneak attack once 24 (average 16) Assume arcane archer for an additional 24 damage (average 16)

224 damage 1st turn.

Theoretically a +3 hand crossbow flametongue could add about 135 damage.

This assumes absolute optimal dice rolls with everything hitting and rolling the highest numb, with a surprise round.

If they won initiative you COULD add 279 in the next turn with these assumptions (but they'll need to roll these crits).

For a total of 638 damage (really generous DM giving a flametongue) . 388 is closer to what it'd come to assuming a +2 weapon and optimal rolls.

Realistically, this will be once per short rest. It requires an interesting reading of the rules. Things must go perfectly with no misses and perfect rolls.

This assumes fighter 5 (arcane archer), Rogue 3 (Assassin), Ranger 3 (gloomstalker), 20 Dex, Sharp Shooter Feat. This assumes Dice rolling for stats as a v. Human.that got a 17.