Dungeon World is an amazing game breathing life into a genre that Wizards of the Coast have done their best to squeeze the life out of. There is, however, one area what DW is, to be frank, ass backwards/completely wrong - and that is the are of HP and the philosophy surrounding them.
"The best answer here is that fighting a dragon is harder because the dragon is fictionally stronger. Just stabbing a dragon with a normal blade isn’t hack and slash because a typical blade can’t hurt it."
I totally get that. Dragons and fabulous monsters are dangerous because, fictionally, they have extraordinary powers and abilities. Dragons can blast you with fire, fly, and are big enough to have lots and lots of reach on you. Thats what makes them dangerous, powerful, and interesting to fight. But lets really cash out that example. Rogar the fighter and his daring company, in an effort to defeat a terrible and powerful dragon, quest far and wide to assemble the ingredients necessary to enchant/craft/find a blade potent enough to pierce the dragons plate-mail like hide. No normal blade could POSSIBLE harm this great and terrible beast... but now, after a long and perilous journey Rogar has a weapon that... will STILL kill the dragon in one only marginally lucky hit. Now THAT violates the fiction of MY world. That would be like me going up to a whale, stabing it with a stake knife, and it dying instantly.
The fictional powers of monsters make them interesting and terrifying combats but in a system where HP exists AT ALL ultimately fiction and math will collide. It simply makes no sense that a terryfing and epic beast can be slain in essentially two shots - after all the questing to discover a weapon that will harm it, after all the trouble and the zig-zagging of even approaching the beast you're telling me that I'm going to drop such an enormous creature so easily? I think we're off the rails here.
HP should be an abstraction, a measure of how much punishment a creature can take - how many mistakes they can make before they fall. Surely, a tough as nails creature is capable of surviving several, if not tens of blows before falling. Dungeon World currently does not have HP system that supports this idea - it violates its own principle. A fictional dragon is going to survive plenty of blows before it goes down (Unless your magical weapon kills it instantly, which, IMO, is a really shitty/anti-climactic fiction).
So what do I do about it...? I just use the HP listed in the 3.5 D&D Monster manual. Creatures like gnolls, goblins and orcs have very similar HP and other monsters my players are interested in fighting stand to present a battle where more than 2 blows are traded with the BBEG. Everything else about dungeon world is rock solid, IMO. This is just one house rule that we use at my table and the reasoning behind it.