Life already has a spider... maybe :earth :gravity or :time would work though.
I'd put it in :darkness at -1 cost. Thematicly suits creepy crawlies, closer to Flesh Spider, could work out a spiritual quad with mark of air
In Life it's too much a "Buff Giant Spider" rather than "Buff spiders" as Giant Spider is designed to be a good creature without web (whereas Flesh Spider is plain bad). If you really want it in element with a spider, go with :death