While playing Reforged mod, I wanted to do some shield training and thought that Juggernauts would be suitable sparring partners. They hit multiple times and they hit hard, so that Bishops and the Bard can develop their areas of expertise, too. Since they can be dangerous opponents, playing was not boring (Here I must recommend Kaucucovnik's Wiz8FastAnim.exe; it really makes encounters faster). I would perhaps let Vi fight, her stats aren't initially very good.
But I don't have much patience for just training. Three points in Shield skill seemed to be a maximum for me. Besides, a shield seems not to help much against the worst monsters in the mod.