Gogo power within + red tearstone + cursed + dusk crown ring --> sniper xbow.
Line that up carefully with binoculars on the stationary ganker waving at you in the forest, and laugh if you get a headshot. I have never tried this, but it would work in theory.
Seriously, they need to make xbows aim-able. I am happy that at least in Dark Souls you could upgrade them and they could be used somewhat; in demon's souls the xbow was 100% worthless for anything but pulling.
as for mega crossbows, I believe the Chinese had ones that had to be spanned using the archers legs to push the string into position
Historically, the strongest 1-man xbows require that + cranking it to pull the string back, then setting the bolt. The reload time was immense, but the force was great enough that it could pierce through chainmail and most other light metal armor and kill someone pretty easily (not so good vs true platemail or lamellar).
A faster and still plenty-lethal version was the one you stood on and used both hands to draw, then set the bolt. That loaded pretty quickly by comparison, but still longer than in-game versions. This is probably like the xbow version of a greatbow and should be used.
Lighter crossbows didn't have the same kind of force. Chinese repeating xbows are quite old actually and had "clips" of like 10 bolts (!), but traded away so much power to be able to shoot rapidly that they couldn't get killing force on their own (these bolts were typically poison-coated).
There weren't many practical versions like dark souls represents, where the reload was so fast and relatively simple. By contrast, however, most xbows could put forces well beyond anything a bow would achieve, excepting the longbows where people trained basically their whole life to use them (those had more range).
Xbows, like spears and perhaps even more so, are among the most heavily underrated weapons in gaming when compared agianst what they actually did in history.