I found the shadow copy very useful, especially because the first time it spawns it also gets a shield...a copy of yours. With the voodoo shield, that meant that the blood guardian couldn't do very much damage (he kept getting past my shield before I got past his, though) and in the other levels, it tends to dodge a lot, probably because I had it on level 2, so it could dodge more than me. It actually did decent damage, but it was mostly sword/magic fodder.