Stuck in tier 17 :/

uh . . . no?
It shouldn't be that hard to get into tier 18, and my first merged unit for arena was at tier 19, if memory serves.

It sounds like you're stuck. And like you typed something wrong.

I made it to tier 17 back during the Celica banner and was stuck there for 1 week. I had a team of three 5* units and a 4* bonus unit all at level 40. Then I merged one of my units (linde) to level 40+1 and was given the gift of having my 5* Olivia as a bonus unit so I managed to climb to tier 19 with a team of Olivia, Linde+1, Fae, and Setsuna all at 5*. Then I had 1 week in Tier 19 with this team and realized pretty quickly that I would not be advancing anymore so my goal now was just to collect all my feathers and not to go all out anymore until I can build a stronger team with a higher bst. I finally got Effie 2 weeks ago so I'm going to try for an armoured team. I should have something at least semi-respectable within another couple of weeks once I can get more feathers. This week I'm at tier 17 again.

Whatever your question was, I hope this helped.

get attacked, you receive extra points for that... i use cavalry emblem and i entered tier 19 xD
but next, i will fell to 18 kkk

It's definitely not... I just got into Tier 18 this week. I think it's a matter of having decent bonus units with high BST to raise your score (it makes a huge difference!). Also play all advanced and go for deathless streaks.
If you want to raise your score/tier in the long run, I would invest in 5*ing and totally building the starting trio (Alfonse, Sharena, Anna). Not only does this raise your score by giving you higher BST, it gives you decent enough units to work with instead of just deadweight, making deathless runs much easier as well.

I've hit tier 19 with borderline tier 20 without merged units a couple times. Like, borderline borderline