The thing about Dungeon World is as long as everyone has similar bonuses to their dice rolls, all classes are equally as powerful. Why? Because they all have an equal chance to A: Solve a narrative problem, B: Introduce or escalate a new problem, or C: Do both at the same time. I feel like it's a really big deal to you that the druid is solving problems you're introducing, but remember that you are a fan of the characters, and you can always turn the volume up (because if there isn't any danger of escalating, why are you even rolling rice?) in response to their actions. Unlike, say, video games, Dungeon World DMs do not (and should not) have the power to create unsolvable problems (or impassable walls). On the note of your other two examples, I let my Bard use his charm on some skeletons, as well as an entire horde of goblins in order to distract and delay them, entirely because his narrative descriptions sold me on it. I also nearly killed him several times when he attempted similar stunts, whenever his poor rolling allowed me to. I also have let (and plan to keep letting) my fighter kill enemies in a single blow when the narrative encourages it, even one that could've been considered a "boss monster." Why? Because he (and everyone else) enjoyed it. There are always more monsters.

So what if my druid has saved the day before by turning into a whale in mid-air and crushing some obstacle? She also has failed rolls to do just that and ended up landing on top of her allies. Personally, I think it's AWESOME that I let her turn into a giant eagle and carry people around, because I can throw "physical obstacles" of epic scale at my players without worrying about whether or not it's an unsolvable problem. And I always know how to ramp things up. I want a wall that's a challenge to get over? Well, sure you can fly your allies over, but you can only do it one at a time, and each one is a single use of your hold, so you'll probably have to roll multiple times (and risk failing), and if the orcs are right on top of you and the building is about to explode you may be able to afford the time necessary to gently airlift each ally one at a time.

So, no, we're not telling you to give up on physical barriers. We're telling you to give on the idea of having "barriers" at all, because an impassable wall is not a threat, it's a decree. Dungeon World works because of the constant rhythm of player moves and DM moves, and as long as you let players have their successes when they've earned them (via the dice and narrative), and their failures when they're deserved (also via the dice and narrative) nobody will be "too powerful" because they will all alternately be solving and creating problems. It's not your job to say which problems get solved and which don't, it just your job to supply (or better yet: discover) the problems in the first place.

I'd say that things are much more tense when getting a solid blow on something hurts, no matter what. I don't care if you're a dragon or a goblin, getting stabbed in the heart is not something you brush off. The thing about some enemies is that getting those solid hits can be very, very hard. Hack and Slash is when you are "trading blows" with a monster, and that 16hp dragon is the living incarnation of terror and destruction—if you're awesome enough to trade blows with a several-ton mass of fire and foot-long claws, then you should be killing it in a few hits. And if not, then you'd better start looking for arrows-in-the-gap solutions, because you're not going to convince anyone that a low-level fighter is just going to stroll up to a dragon and spar with it—even the act of getting close to it is defying danger in like four different ways.

Increasing the difficulty of a monster is less about mechanics and numbers, and more about using the fiction to prevent the PCs from just saying "I attack it". I mean, a demon? If you want that to be a scary fight, go on the aggressive with the fiction: force the players to respond to the terror and brutality ("as you charge the demon, necrotic power pulsates heavily from it—even getting close to it will be a challenge" or for the wizard "as you tap into the magical energies around you, you feel that the very essence of magic in the area is completely evil—manipulating it may be putting your allies and your sanity at risk.") so that they have to earn the right to hack and slash (or if they're particularly clever, gain enough of an advantage to circumvent rolling altogether.)

Going back to Middle Earth, not only did Smaug die in a single arrow, but the Witch-King of Nazgul, one of the most feared servants of evil, was slain in two blows, and Shelob, a manifestation of complete evil and gloom, was sent scampering by a blow to the stomach. In all of these situations, the hard part was not "dealing a lot of damage" but even being allowed to deal damage in the first place.