I am level 17 or 18 and when entering kvatch oblivion gate there are those giant lizards and flame ladys and that is not only on the oblivion gate but also in the open world.

Now each of them is at least 5 minutes to unless they do not kill me first!
And when entering the kvatch gate there are 4 monsters jumping on me!

Is it normal? because I saw someone in youtube that entering Kvatch gate and he has an ebony axe and there are scumps attacking him and he kills them in 3 hits!

Is it normal that the game is so difficult on level 17? or maybe something wrong with my game?

By the way I am playing on medium difficulty and I am not using any mods.

Yes it's normal. The scamps are at a lower level and then easy to fight. At your level the monsters also levelled up. Maybe you are not well prepared. What is your status actually? What is your weapon and armour?

Dwarven set of armor and elven sword and dwarven shield.
Not enchanted.

Try getting acess to the mages arcane unirversity and enchanting some equipment, maybe go with absorb health or make a spell weakness to magic 100% to amplify your enchantments, use grand soul gems to enchant.

Well my warrior Redgard at level 18, just after Kwatch, is holding a Katana enchanted (Fire 5 points) level 9. He fighted at Kwatch with the Katana one hand given by Jauffre along with other armours and weapons. I remember that there were even a two handed one. Did you ask Jauffre for assistance? He will open a chest and tell you to help yourself. The same for the others. They will give you something.

My warrior status was: Strength 83 Intell. 82 Vol. 59 Agil. 47 Veloc. 50 Resist. 84 Pers. 32 Fortuna 50. (Of course at level 18).

Maybe you need more strength and resistance at level 17. You can compensate by enchanting items.

Hope I helped!

You can enchant with common or greater soul gems. Just 5 points on a blade is already something and it last longer....