Azato,
We've all played ages of video games and 'classic' RPGs (with the classic fantasy tropes) where we're taught that fighting the monster is a matter of just doing enough papercuts that it falls down while living long enough to do so (the WoW or Final Fantasy model).
But in Tolkien Smaug wasted a village, killed thousands, but was killed by a single arrow placed correctly in a missing scale.
Think of these fights more in terms of literature and pacing instead of the classic 'they have X hp and we have to swing Y times with Q hits to drop it'. The problem in this context is that there is no accounting for fiction, this is a mechanical solution (a simulation) of a sword doing consistent damage, and scaling monster HP to allow for the same tool (swing) to be applied to every problem (monster).
I had this problem. I did a quadruple take when I read that a DRAGON has 16 hit points (a level 1 ranger can do that on a max damage roll). However let me describe a fight to you and maybe this will give you the 'inkling' of what's happening.
So the party needed a magic item, and they researched and found that a hero wielding said item was slain by a dragon. They get some info from a different dragon's drake-in-human-form servant, and go and steal said item. Remember, magic in this world doesn't mean 'magic' in the +'s sense, but this spear can pierce souls and is thus necessary to defeat a sorcerer king. Ok, so we have a very angry dragon about to attack something. 16 hp again - ready?
The party is riding back into town ready for a nice hot bath, some resupplies (their rations were running low), and a re-focus on hunting down the sorcerer king. The moon goes out for a second, they feel the wind shift, and then something lands on city hall with a massive crack. They have a few seconds to blink before they see a serpentine head snake down and shred a guardsman in mail in a single hit (announce future badness, this is the 'messy' tag). They kick up the speed and head towards town. I plop down paper, and quickly draw some snaking streets, sketch out some boxy houses, plop down a big die to represent the dragon. As they're about to walk in, I pick up a handful of red tokens, and describe the inhalation they feel from this far, and the words in dragon-speech, and basically drop a pile of red on town and explain it's on fire and how the flames themselves are being shaped and commanded by the dragon.
Their horses freak. They manage to get off (a few taking some damage from a panicked horse running and one being hit by a branch). They start advancing through this hellish landscape, where an inconsistent shadow would swoop down and split someone in half, and people burning to death beg for mercy and help while holding swaddled children turning to ash in their arms.
The group starts to help the townsfolk (this is not a magical node, so the wizard can't just ritual up some rain) when a building shatters with the landing of a 4-5 ton creature, and it opens up it's pipes, it's golden eyes burning and it's metal hide resonates with a roar (terrifying).
Their charges scatter, the PC's have to defy their own terror to attack the thing. They do negligible damage (yay 4 armor) for those that DO anything, and realize that the only person who has a shot at killing this is the armor-penetrating wizard spells. Unfortunately, so does the dragon.
What ensues is horrific. One fighter takes up defensive position, when the dragon strikes it doesn't just do 1d10+5 damage, it rips off his arm (messy remember?) and shreds mail like tissue paper. It does breath weapon attacks that cause ALL of them to defy danger or burn.
The party breaks and runs. The dragon laughs and settles to ash the village and eat any survivors.
The Dragon had 16 hit points. The party did 9 to it before they left. And when I said left, I mean they ran like rabbits into the night with few provisions, no easy means of recovering them, and no thoughts in their heads other than survival.
The moral of the story is it's not about the hitpoints. In my 4e game the party had a dozen dragon kills under their belt. The dragons were mechanically threatening, they were tricksy, they were tactical, but their claws and teeth didn't do damage, they did numbers. After this session they explained that they had never been so scared of a monster.
Make the fights epic. Use the fiction. Describe their skin curling black from fire. The bones shattering from the unyielding stone grasp of the earth elemental. Most fights clean up the fiction by saying you take 5 damage. Make it stick, make it hard to heal, make them scarred and battle hardened having earned every mark, and every wound a story.
You don't need 2500 hp to make a fight scary or hard.