Every Aura spell requires a target (C.R. 115.1b), so that if you control Willbreaker or Cowardice and target a "creature an opponent controls" or a "creature", respectively, with Sluggishness, Willbreaker's or Cowardice's ability will trigger (compare with Silverfur Partisan).
Likewise, for purposes of those two abilities, even a mutating creature spell such as Archipelagore is a "spell or ability" that can target something (C.R. 115.1e).
EDIT: Correctness edit after comment 3 was posted.
EDIT (Apr. 15): Edited.
To be very precise, we'll say that Aura spells do target.
An Aura targets for only as long as it's a spell; once it has resolved and attached itself to some other object, it no longer targets; it becomes 'attached' to the other object.
This distinction is important because, while Shroud can stop an Aura spell from being cast, it cannot later remove an aura which has already attached.
303.4a An Aura spell requires a target, which is defined by its enchant ability.
303.4b The object or player an Aura is attached to is called enchanted. The Aura is attached to, or “enchants,” that object or player.
More specifically will a card like sluggishness target for purposes of willbreaker or cowardice?
Likewise, for purposes of those two abilities, even a mutating creature spell such as Archipelagore is a "spell or ability" that can target something (C.R. 115.1e).
EDIT: Correctness edit after comment 3 was posted.
EDIT (Apr. 15): Edited.
An Aura targets for only as long as it's a spell; once it has resolved and attached itself to some other object, it no longer targets; it becomes 'attached' to the other object.
This distinction is important because, while Shroud can stop an Aura spell from being cast, it cannot later remove an aura which has already attached.
303.4b The object or player an Aura is attached to is called enchanted. The Aura is attached to, or “enchants,” that object or player.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules