I do not see this enchantment to be a good enchantment at all when you can get sharpness 5 on a sword instead and still one shot all the spiders. Instead, why not add like a really op spider boss that can only die to that enchantment? Are you guys planning on adding another boss?
While I agree Bane of Arthropods needs a buff, I think adding a whole boss for it is kind of extreme. Regardless, if you want to suggest a new boss, you need details, such as health and behavior. Also, Mojang doesn’t read this forum.
Since 1.9 you can't one-shot spiders with Sharpness - a Sharpness V diamond sword only deals 10 damage compared to 14.25 before, a 30% reduction in damage, and even then it could only one-shot cave spiders without a critical hit (before 1.9 you could even one-shot silverfish with an unenchanted diamond sword, which added 7 damage to your base damage of 1; now it simply replaces it for a total of 7 instead of 8 damage. Swords also worked the same way prior to 1.6). Critical hits multiply the base damage (including unarmed damage before 1.9, but not enchantments) by 1.5 for a total of 18.25 damage before 1.9 and 13.5 since, which is below the 16 damage needed to one-shot a normal spider.
By contrast, a diamond sword with Bane of Arthropods V deals a whopping 19.5 damage, and even level 2 deals enough damage to one-shot cave spiders (+2.5 damage per level vs +0.5 plus a base of 0.5), and level 4 is enough for regular spiders (so the main issue here is the usefulness of level 5 enchantments,. which should have some clear advantage due to the difficulty of obtaining them. Some spiders do spawn with Resistance but only on Hard and with a low chance; this should occur on all difficulties with the chance based on regional difficulty so that it is at least as common on Normal as it is on Hard right now).
IMO, it is Sharpness that needs a buff; instead of making it add (0.5 + 0.5 * level) damage have it add (0.75 * level) damage so Sharpness V adds 3.75 damage instead of 3 for a total of 10.75; this enables it to kill zombies in two hits instead of 3 (this is due to their natural armor, which gives them 20.3-21.74 health depending on armor penetration). Similarly, Smite was nerfed from being able to kill skeletons in one hit to 2 hits (from 20.5 to 19.5 damage; before 1.9 zombies needed 21.74 damage to kill so it never could one-shot them without a critical hit. That said, Mojang released a combat test snapshot that increased sword damage back to 1.6-1.8 levels so if this is implemented in a release it will buff both Sharpness on zombies and Smite on skeletons).