A villain is wreaking havok in town. He can become intangible at will, making him invulnerable to all conventional weapons and the hero team's powers.
However one of the heroes notes that since he's not invisible (only slightly translucent*) he must interact with light somewhat normally. That hero then suggests building a laser gun to try and kill him (capturing or containing him is now off the table). But a laser isn't the best choice, is it? Any harmful electromagnetic wave will do.
Assuming the villain interacts with light like a normal human being, what wavelength should our heroes go for?
As far as I know, you would need a huge installation to kill a human with a laser. Gamma rays are much more dangerous but possibly hard to produce.
Assuming current technology. The ideal size of the weapon would be between a handgun and something than can be mounted on an military LUV. The heroes can work with the army to build the weapon.
The only two things to take into account are:
- Harmfulness of the wavelength for a human being
- Practicality to produce that wavelength in a lethal amount
(*)The villain is only slightly translucent, in a way you can tell if he's currently intangible or not. Only a small portion of light goes through, about 95% of visible light interacts with him normally. You can assume the same for other wavelengths.