The narrative at hand is a perfect reason to make things difficult for the player. You should always ask questions like crazy, ie. ask for justification from the player about how he does what he does. He may want to hack and slash at the troll but he doesn't decide on what move his intended action corresponds to. Feel free to challenge his intent by clarifying details and then asking.
I can imagine a conversation going like:
— I hack and slash at the troll!
— Cool, how do you do it?
— I just swing my sword to chop his head off!
— Well, he's swinging a huge tree trunk around, how do you intend to get close enough to bring your steel toohpick in striking range?
— Well, I try to look for an opening and dive in.
— Looks like discern realities, please roll+wis.
— That's an 8, I get to ask one question, ok, what should I be on the lookout for?
— His swing has a semi-random looking pattern but you can guess it out.
— So I can just run in with perfect timing to avoid his swing.
— Cool, that looks like defy danger with roll+int to outsmart the troll.
— What? Not dex? Why?
— Because you can see that even with speed and reflexes, you'd eventually get hit by the trunk unless you figure out his rythym.
— Well, here goes… a 4, ouch!
— There's no way you're getting in by timing it, sooner or later that trunk will push your skull into your ribcage unless you keep your distance. That's what you have been doing instinctively anyway without realizing that you have been cornered, with nowhere to retreat anymore. The troll seizes the opportunity to deliver its skull-to-ribcage promise by swinging the trunk straight down on you. What do you do?
— Whoa, I duck and tumble inside his guard.
— Sure, defy danger with roll+dex…
— 9 this time. What now?
— Well, you're obviously not going to pull that off with a sword in your hand. Either let go of it or feel the impact of the trunk on your back, your call.
— I need the HP, so ok, I let go of the sword and tumble.
— Cool. You're now inside the troll's guard without your sword. The troll grins to show you his rotting teeth, drops the trunk which just falls on your sword, and lunges to grab you by your throat. What do you do?
Here, the GM diverts the player's action so that he's on the defensive rather than the offensive he intends. The player had to go through three potential points of failure and he still couldn't take a real swing at the troll.
That, in my opinion, is what makes this a difficult adversary. Not how improbable your chance of succeeding is, but how many other things you have to go through before you can take a shot at your quarry.
Regarding your player's questions
Can't I try to dodge the attack?
You probably already have done the show an approaching threat move and told him that the troll is swinging a tree trunk at him, asking "what do you do?" right? Then he probably said "I try to tumble and roll to dodge it", which you interpreted as the defy danger move based on roll+dex, for which he rolled miserably. That's why it's ok for you to just deal damage, a hard move, telling him what horrible thing happens to his bones.
Why do I get damage automatically and why does it pass my armor?
You don't get damage automatically, you do get damage because a troll hit you with a tree trunk, which you just failed to dodge. The impact gives you a concussion and twists your joins in unnatural ways, that's why your steel suit which is good for keeping sharp edges from cutting you doesn't really help much here.
In other words, give him details that would make sense, and only then tell the corresponding game mechanical impact.