Context: a group of men are walking down a secluded road, they get the feeling they're being followed and sure enough their hunch is proved right when they're suddenly attacked, a projectile is launched at them but they manage to dodge it (at this point their attacker is still hidden).
One of the men tries to provoke this hidden assailant to draw them out by calling their attack weak and sluggish, he also says this:
What I'm not sure about is "こたぁねえ" is this a contraction of "ことはない" in which case the speaker would be saying:
Whether or not you believe it, it's not like that was a lethal technique just now was it?
Or if this is totally the wrong assumption, what is the correct way to parse that sentence?