Hmmm, what does it do to anti-mattered creatures? Does it work on weapons? What does poison count as? Also, What happens when things like venom occur (Arsenic and Puffer fish)
To answer your questions:
No. Attack decreases to zero and then stops.
Therefore antimattered creatures being below zero would not be affected.
Whenever a creature attacks, it looses one attack power.
This focuses on creatures so unless you have animated your weapon and turned it into a creature it would not target them.
Poison is a spell and not a creature so as above.
Aresenic again is a weapon so unless it is animated it would not be able to be targeted by this card.
Puffer Fish posion on attack so if their attack goes to 0 they do not attack therefore they do not poison.
Thanks
Scyther