Corwyn:

Readying his Flaming Greatsword, Corwyn casts Mirror Image just before the battle begins!

Initiative:  15

Lucas: 

Initiative: 18. (Just in case it comes up matter)

Passoveloce initiative: 25

Assuming that Passoveloce gets to go before the creatures, he fires an arrow at the spider: Attack: 13 Damage: 19  (includes sneak attack: assassinate and sneak attack bonuses)

Passoveloce fires a 2nd arrow at the spider (assuming the first attack missed): Attack: 17 Damage: 10

(If the first attack hit, he still has advantage on this attack, the second attack roll (for purpose of advantage if necessary): 13)

Passoveloce then moves back through his fellow adventurers back into the hut, stows his bow and uses his bonus action to draw his shortsword.