I managed to get him using the mouse too. The trick seems to be that you don't need to aim DIRECTLY at Count Etcheverry. Rather, you simply need to line up the spyglass so that Etcheverry's head is in between the "I" and "II" lines. This gives you the 1.5 degrees needed to hit him. (Or rather, smash the platform he's standing on to splinters, which also kills him.)

Is it actually possible to land a direct hit on Etcheverry? I've never managed to do so, and my current solution works like a charm, so I've never tried investigating further.