A good game should not be difficult. It should be effortful. The idea that by doing enough tasks, puting in effort, and working hard and efficiently, you should be able to conquer the game. What make castlevania, dark souls, super hostile, and touhou fun aren't their difficulty, but that their core mechanic is that by puting in time and effort into memorising enemy patterns, you can surpass your foes. This effort is not about grinding (unless the grinding is made to be fun), it's about thinking, planning, and observation.
Enemies that, by their very nature, surpass you in speed and damage, that attack you on their terms are not fun. They do not have critical weaknesses that you can exploit to kill them. You literally have to hide and only venture into the world far later in the game when you can even remotely handle them. You have to hide until you can face them with superior gear.
My suggestions are as follows:
1. If you can't beat it with no armor on, it shouldn't attack you. Enemies should not try to kill you with no chance of survival.
2. If it's an enemy in the vanilla game, you shouldn't mess with it's speed or damage output. Mojang balanced that for a reason.
3. Lowering one's hearts increases the difficulty, not the effort. If you want to keep this mechanic, you must be able to gain hearts by quests, by crafting, or by spending exp. Three things that the player has direct control over.
Read this: http://lesswrong.com/lw/ww/high_challenge/
Then some of these: http://lesswrong.com/lw/xy/the_fun_theory_sequence/

