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.

Some initiative rolls
Paul = Unable to parse dice roll.
Toruun = 8
Spider Dragon =21
Medium spider = 22

Initiative: Passoveloce; Xx@^$!; Ker-arach; Lucas; Corwyn; Paul; Toruun

1) As the party gets ready to put their plan into action, to their surprise there are now four versions of Corwyn all doing the same thing and a small aperture opens in Toruun’s armour and a tiny winged creature flies out and flutters near Toruun’s head.

2) At Passoveloce’s command, Lucas pushes open the double doors and the Passoveloce, Corwyn, Paul and Toruun move to their assigned positions - Grids ‘A’, ‘B’, ‘C’ and ‘D’ respectively.

3) Passoveloce snaps off two shots at the spider but the webs its is hiding in provide a degree of cover sand both arrows are enmeshed in the webs. He then scuttles back to the hut behind Lucas and then stows his bow and draws his sword.

4) Corwyn, Paul and Toruun find themselves enmeshed in sticky clinging webs (it also includes Grid ‘A’ where Passoveloce had been) - DC 15 (Strength {Athletics} to break free!

5) A beam of darkness spear out from the spider towards Corwyn and his images (3) {3 rolled} hitting one of the duplicates and destroying it. A second beam of darkness spears towards Toruun Attack: 23 Damage: 17. {acid damage}

6) A cloud of small black dots emerge from the spider dragon’s mouth and flood towards the party. When they arrive the party can see that they are actually small black spiders. Everybody - except Passoveloce - must make a DC 15 Dexterity save or take 35 piercing damage and 32 poison damage - half for successful save.

7) Lucas you’re up followed by Corwyn.

By the way Toruun’s homonculi servant finds itself also covered in small black spiders and crashes to the ground unmoving!

Paul brushes spiders off himself muttering something about damned bugs (DC 15 save required and 26 rolled)

(OOC: I believe Lucas should also be safe since he cannot leave the dome without dropping the spell in the process. But geeze louise, that's a lotta damage! And spewing spiders? Freaking Awesome~)

"What the dig?! Did the dragon do what I think it just did? Aww, shoot either that's a ye olden time's dragon or some new crazy hotness they've cooked up!" Lucas comments as some spiders splash against the barrier. "Either way, I think you still have it Passoveloce. That was some sharpshooting, and if you angle a little closer to the throat, you'll really have it. So, keep at it!" He encourages the halfling while fishing around in his own pocket until he produced what looked to be a sliver of glass.

Then much like a conductor's wand, he waved it about while singing of a yearning for destruction, conjuring a cloud made of shards of glass and sending them flying through the barrier to engulf the spider. 

Bonus Action: Grant Passoveloce a Bardic Inspiration(d8)!

Action: Cast (5th level)Storm of Blades in the same space as the beam blasting spider, inflicting 34(29) Slashing Damage.

OR(If the beam blasting spider would be further than 60 ft, but the dragon is not, targeting dragon instead. If both are outside of 60 ft, but are within view for Lucas, see spoiler. If neither are in view after unleashing their attacks, see same spoiler for Lucas' alternative action)

(OOC: Just gonna correct the damage from the spoiler option here.)

Corwyn:

Somehow managing to use the edge of the dome to help him momentarily avoid the spiders, Corwyn dodges the worst of the incoming creepy breath weapon (Dexterity Save 22).  Easily working his way through the webs (Athletics check 28), Corwyn moves to the side of the spider dragon so that it cannot both envelop him and the others with its breath weapon at the same time.  He then starts an impressive attack routine against it with his flaming greatsword (rolled below).

Flaming Greatsword Green Flame Blade Great Weapon Mastery:  Attack: 18 Damage: 26

Iterative Attack #2 Flaming Greatsword Great Weapon Mastery:  Attack: 27 Damage: 15

Iterative Attack #3 Flaming Greatsword Great Weapon Mastery:  Attack: 22 Damage: 23

Bonus Action Second Wind to heal:  16

(If there is an enemy adjacent to the spider and Corwyn and he hits with that first attack, the Green Flame Blade does fire damage to it:  14 )

Corwyn:

(Assuming at least one of those attacks hit, Corwyn applies the "Sap" condition to the spider dragon, giving it disadvantage on its next attack.  All his attacks were against the spider dragon, with only the side effect from the green flame blade affecting the spider if it is close enough).