Help me building a team please ^^’

Summer Cordelia (SCordelia)
Black Knight (BK)
Alm
Nowi
You have 2 offensive units in SCordelia and Alm, and 2 tanks in BK and Nowi. If you don't have Alm's special refine on his Falchion, he is not worth using. He is pretty bad without his refine.
Nowi's builds are expensive but extremely strong.
Make sure the IVs on your units are good.
That's just a quick and simple summary of what you should do. If you want help on builds, just speak up.

I suggest you find the guides on building arena teams and read a little about roles.

Just to give a summary of what you have there:
Tanks: A.Tiki, Nowi, L.Ike, BK, maybe Alm
Duelist: S.Cordelia, BK in certain builds, possibly Alm
Support units: Elise
Glass cannon: Olwen

It's ok not to have a glass cannon (though for arena defense I would recommend them; the AI is very aggressive and easy to bait; may as well throw something at people that they will have a hard time tanking) and usually OK not to have any dedicated support units. It's hard to play without tanks, especially PvE (as opposed to arena), but it can be done.

Have you inherited any skills onto any of the units you mentioned?

How about your play style? It seems from your list so far that you prefer to play units that can withstand a lot of enemy attacks. Have you tried units like Nino who can't take much damage but who hit super hard? How do you like them, and how do you like the different movement types - infantry, armour (BK can only move one space at a time), cavalry, flier?

The only unit I think is good for arena is Nowi and Tiki since they come at 4*, you can easily merge them, that is when you said serious team for arena, you want to hit tier 20 and stay there, you will focus on 3 characters, 2 of them being Tiki and Nowi, merge them to +10 as priority, the 3rd character in the core should be a green unit that has decently high BST, and your last unit is the bonus unit. You can use this core for all arenas from there.