I've entered the ruins east of Riverwood on the mountain, I think they're called something like North Skywatch, and although I've killed the bandits inside and outside with great ease, there is a Giant Frostbite spider inside. I'm a level 7 sword+shield (regular steel) with destruction magic wielder, and although I've got the Firebolt and 1-handed at 31 (+20% perk to damage), my attacks do very little damage, so fighting it would be just a matter of hit-and-run+potions matter.

So I'm just curious - was I not supposed to go there yet or are they always so strong?

just use a shield i mean seriously they are easy, more easy than a Bunny

i did, and still its attack leave me at 1/3rd of my health, so I have retreat after every one of its attacks...

Turn the difficulty down?

But that wouldn't be the point of the game, right? After all, I'm playing on the default difficulty wearing heavy armour. So it should be either possible for me to defeat it now or later.

I don't know why, but some times a spider or whatever seems to be extra strong, I one shot some then I get one, same size same level that acts like I'm hitting it with a marshmellow. Try to long range him or leave and come back with more of everything.

Giant Frost Bite spiders have either 380 or 510 hit points depending on color, they deal 45dmg per bite with another 10/dmg for 3 second in poison. He's level 7, its not surprising he lacks the skill and equipment to take one on.

Well, in the end I did manage to get him by employing the tactics "Hit with a Firebolt twice, run back into the passage where it doesn't follow and come back". Though it doesn't seem exactly like a fair fight...on the other hand, would a spider be able to doors or not? Though it's living in a corridor with doors on either side, so it must have somehow gotten in, unless the doors are always open when somebody stumbles in...

Those statistics make sense, so that's why the spider was always able to deplete almost all of my health with 3 bites.

If that's the spider I think it is I also found it quite difficult to begin with, but the way I defeated it was to run in and attack it back off close the door until health/stamina returned, or used a few pots then open the door and smack it around a bit again and then retreat if it isn't going well.
if it isn't that spider best option is hit and run

Giantfrostbite spiders are a sort of minibosses. Due to their size however, they can't go anywhere, especially in tight tunnels. 'hit and run' tactics are the best at low levels unless some sort of ranged attack (magic, bows) is available.