Saint Privatus (?)
Euphrasian Basilica, Poreč, Croatia
Several male Christian martyrs are said to have been beaten with cudgels.
The mountain redoubt seems to me to be the strongest evidence for this saint's identity, and given also the gray beard I would guess that this is St. Privatus.
St. Timothy, the friend of St. Paul, was beaten with cudgels and died of his wounds, but he is almost always portrayed as a young man.
St. Domnius was stripped naked and beaten before being beheaded; the few pictures I have seen are post-medieval, do not show him as an old man, and make no reference to mountains. The fresco's saint has indeed been stripped down to his undershorts.
St. Maurus of Parentium (Poreč) was beaten with fustibus nodosis, "knotted cudgels" (Deperis, 32), which the cudgels in the picture seem to be, and then tortured to death in other ways.
St. Privatus, martyred bishop of Mende, France, is the only one of these four who has a gray beard in any of the images I have seen. Also, his story is the only one that includes a mountain redoubt such as we see in the background of this fresco. When the Germans invaded Gaul, he and his people took refuge in a mountain fortress (ad montem Gredonae quasi ad praesidium confugerunt, Acta Sanctorum, August vol. 4, 438). But Privatus was captured and tortured to death, the first torture being a beating with cudgels.
The date of the fresco is also a problem. Most of the other frescos in the basilica are of a style that one would readily assign to the 5th or 6th centuries, but this one is decidedly medieval, and the torturers are dressed as if they were pages in a medieval court.
This image in full resolution
More of St. Privatus
More of St. Domnius
More of St. Timothy
More of St. Maurus of Parentium
Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.