I've been GMing for a couple of sessions now and I'm still struggling at making encounters or situations more difficult. I will describe dangerous situations and encounters in fiction but the mechanics make it too easy for my players to succeed.
For instance no matter how big I describe the Troll, if the fighter of the party charges him and swing his sword, roll+2 for Hack and slash, he'll easily hit the troll. Almost 40% chance to hit without consequences.
Best Answer
One thing you can do to make a big boss dangerous is just make normal Hack & Slash useless. Imagine they are fighting a storm giant or something the like, and they normal Hack & Slash, you can just say "ok, you are just chipping his toenails, that is not going to work."
Force them to be creative: climb the giant (defy danger), try to out maneuver it, use terrain to get higher, make it kneel, whatever. It's a giant, it makes sense a normal attack on his foot with a sword is not going to work (even if it works in other games).
About hard moves, I would say the troll hitting is a consequence of them failing. For example you say:
— The troll comes at you with its huge club, what do you do?
— I tumble to the side to avoid it. (player rolls and fails)
And now is when you decide a hard or a soft move. You could do damage, or you could just put the player in a harder situation (you are not fast enough and the club grazes you taking you out of balance, your weapon goes flying and you are now lying on the floor, while the troll prepares to step on you!).