Long boss battles are good, but then the boss battles in DD were pretty long, IMO. A low HP boss with better use of health potions sounds great in theory, I agree, but there's always going to be some sneaky player who works out how to take them down very fast. There's also one other problem - a boss that is healing cannot also be attacking at the same time, so that the boss battle may be longer but in many ways it becomes less dangerous. I can't think of that as a good thing.
To me, boss battles can be long or medium length, but the real defining quality is that they should be dangerous. The mark of a good boss battle is how many times you die before getting it right. 4-5 times is a good battle, IMO!
A long fight in and of itself has no merit, to me.
Welcome to the forum, BTW
