Where are the spiders for World Wide Web?

Guardian is the mob type (large mob), Spider is what it actually is.

One possibility for farming is Theroxis (the spider boss in the cave just west of the big pyramid). Theroxis is on a 5 min repop timer so this would take about 5 hours if you stuck with just farming this enemy for the achievement.

There is the spider boss inside the Anhk instance, which is also a guardian spider.

In the Egyptian zone City of the Sun God while on the quest "They Were Beautiful Once" on tier 4 you're asked to kill the spider "Heropsis" who also counts as a guardian spider;

Heropsis is the easiest Guardian its spawns little adds now but respawns like a normal mob so you just leave the room take a drink and go back in maybe 30 sec after completeing the battle.