Spider 15 is force affinity. Scyl could work as "off affinity tank", like Ophelia suggested. Scyl should have good defensive stats (hp and def) anyway. Maybe you should swap one set for this fight and use Resistance gear, to avoid the poisons. You should not bring additional magic affinity champs to the fight.
Stag Knight is a sure pick for this fight. Decrease atk and def on all enemies, the small Spiderlings attack with lesser power and the def of the big one is down for your big hitters.
Coldheart and Armiger as damage dealers and for turnmeter decrease. The big Spider should never get a turn in this fight.
Doompriest can remove one poison and give some healing each turn, her A2 has increase attack for Coldheart.