You're making a wrong assumption here: unbreakable, strong or being an effective weapon doesn't necessarily mean something is heavy, especially when you consider extreme differences in applied force.
For example, look at historical Japan: they used paper to make armors, which were indeed effective versus arrows (something you wouldn't believe when looking at some paper sheets).
But back to weapons: When talking about physical damage, you're always going to measure impact force, or more specifically the kinetic energy of the weapon in motion.
How to calculate kinetic energy?
E = 0.5 × m × v²
As you see in the formula, mass (
m) is only one deciding factor. Indeed, velocity (
v) has far bigger influence:
- Double the mass and you double the kinetic energy.
- Double the velocity and you increase the kinetic energy by factor four!
How to accelerate something? By applying force (i.e. what Thor, Hercules, Amadeus, or whoever do). I'm sure it's save to assume that those guys will be able to apply different amounts of force, which also explains why the weapon obviously gains strength just by being wielded by one of them.
If you want a simple example for this: Take a bullet for a pistol. Throwing it at someone by hand won't do any harm (maybe if you hit their eye or something...). But if you apply greater (i.e. non-human) strength to it by using an actual pistol, you can kill people with it, despite the bullet being very, very light in comparison to other weapons or ammo.
Calculating the actual impact is something completely different (which will also consider things like weight, how brittle something is, etc.). But again, just because something is very durable doesn't necessarily mean it's heavy; think of modern stuff like race cars made of carbon.
As for the actual "agility" part of the question: Well, I guess it's just that classic "hero can do it all" approach - sadly.
Judging your image above, it looks to me as if the artist wanted the reader to see this as a "simple" process:
Amadeus calculates the correct angle to swing as well as the proper velocity vector while swinging (therefore there's that formula stretched around him as well as the "estimated flight path" being visible).
While I could believe this, considering he's obviously some genius, there's one problem here: Hand/eye coordination or more specifically practice. Knowing the correct angle and velocity to throw something is one thing, but actually applying it is something completely different. Something like that usually requires lots and lots of training and even then results might not be that accurate. Think of athletes throwing hammers or spears: It's trivial to calculate the perfect angle to throw something, also adding the perfect velocity if you want to hit some specific spot, but whether they achieve it is some different question.