Originally Posted by Gladiat
Yes, it would do ~300 damage, but you're forgetting the important part. Whirling Sword is in the Sword section, which is all elemental skills with the exception of Thunder Sound. The damage isn't reduced except against people wearing Tactician/Giraffe sets.
Whirlpool Sword is in the Charge section, which is melee. 2-2.5k defense (attainable with AEH/AWD equips) will most likely cut that damage in half, so you get only 150~180 damage. Needless to say, it's basically a low-level killer/equipment abuser killer.
IMO... the all skills in the Sword tab is worth maxing... maybe with the exception of Thunder Sound and nado.
I doubt maxing it is worth it. 150~180 isn't much damage... although it does 5 hits. The decay will lower it to ~40 damage per hit, and you'd have to use it at the beginning of the combo, with 5 hits doing the damage of 3 or 4 hits.
I think they killed off the elemental resistances for those types of armor.