Who did you use for 15 and 16?

I think Ignatius, Duchess and Stag are locks.

I did 17-20 with Duchess Stag Armiger Alure MM. I need to look closer as I don't see a Monster on your roster.

Probably the same core team that beat Spider 16, just try to observe how that void affinity affect the fight and react accordingly by making some changes to the team or the turn order or the equipment.

You have enemy max HP and AoE damage dealers, you have turn meter manipulators, you have AoE debuffs, you have Unkillable , you have ally  reset skills, you have resurrection and they all are 6/60. You have good backup options among your 5/50 like Scyl of the Drakes and Ultimate Galek, even second Coldheart.

You have the tools, but you have to find why you can't do it on auto and you have to make changes according to that observation.

If i may add something more specific about your equipment - i hope the way you have build your Wurlim isn't representative of the way you build all your champions. If your other champions are that slow, that may turn out to be your main problem with Spider. Because you seems to have all the champions you need, but there may be mayor problem with your equipment that block your progress.