I assume the poor sod who was host to the spiders is already dead, so no need to worry about collateral damage:
K:Nature:
Rolled 1d20 : 14, total 14
Storm Walk: Shift 1 square back, target the swarm in G13.
To hit vs. Fort
Rolled 1d20+4 : 13 + 4, total 17
Damage
Rolled 1d8+8 : 1 + 8, total 9
Then move to B10.