You shouldn't rely too much on these kinds of websites. By nature they speak in authoritative-sounding language, but their understanding of the series is no better than yours. The line you quoted is just paraphrasing Schierke's observation in Vritannis' harbor (at the beginning of volume 32) when she sees the giant figure Ganishka has created that's practically a cloud formation. It refers to that instance specifically, but that's not the extent of his power.
What's complex with Ganishka is that he was both an apostle and a magic user, presumably having been taught by Daiba. In the harbor, he was projecting his ethereal body and gathering elementals around it to create that giant form, like Schierke explains. That's why when Guts strikes him, we cut to him screaming in his quarters. It wasn't just his apostle form. However his power as an apostle was no doubt what allowed him to do it, since he wasn't a master magician.
It seems that by default, he could turn into fog and throw pseudo-lightning bolts around. That's what his apostle form was. We don't know the exact details of how it works, no more than for Grunbeld, Locus, Irvine, Rochine, and so on. Just that it's powered by evil magic (and that's enough for the purpose of the story). But with Daiba's help, he could also use his fog to do many other things.
One thing he achieved is producing larger and larger quantities of said fog. He could extend it over vast expanses of land, like for example covering Wyndham in it. And at least to some degree, he could feel who was within it. By getting animals to breathe it in in sufficient amounts, he could also transform them into monstrous familiars, and they could be controlled by trained acolytes if those breathed in the fog as well. It's essentially a bastardized form of magic, likely engineered by Daiba, that relied on Ganishka's power. It made it possible for him to have an army of monsters at his command.
Lastly, with his vast resources he managed to capture many apostles and had them sewn together to create a hideous contraption (which I tend to equate to an "artificial beherit") that was used to produce the Daka. That again relied on his fog to control the process (with a number of acolytes attending to it), but the actual transformation of the foetuses was only possible because of the captured apostles (who were kept alive so that it would work). This again was something that Daiba likely oversaw.
