Get an Ibis before a Maniacal Harm for DT. You want more magic damage (x2) or more magic damage and Bel's Mirth, which you can't get with the gem slots Maniacal Harm has, and a bit higher physical dps won't make a big difference on a DT.
Waning Boundary is the best protection shield indeed, and it's good for PvP too. I use it on my DT but mostly when I swap weapons for more survivability.