Boss: http://mmdks.com/2448
Trick is to get cursed and activate the RTSR. With the Sanctus and your remaining health, your HP will remain stable with PW (well, drop extremely extremely slowly).
For Pre Boss, if you want to use PW (so, have multiple copies), then use the Sanctus as well, instead of the Crest. The damage it does is amazing . . . just don't get hit.
Any suggestions to make it better?