How to pass to rank 19 in arena?

Can I ask what their builds are?

Hello basdax!
First of all you always need a bonus unit to get more points for your arena runs (i think that you already know that.)
After this watch the BST of your units. Reinhardt, Female Robin and Ninian are pretty low so you won't get a lot of points for them. Try to use some armor units or infantry with fury on their A slot. So you can push your points a little more.
Then the costs of your skills. For example: moonbow costs 200 points. Aether 500. Aether gets more value for you points. Rally assists like Rally Spd/Def do the same thing for you.
At last your merges for an unit can get you some points.

The best way is to get high BST units to stay in tier 18-19!

First of all you should at least replace either Ninian or Reinhardt, their class BST penality (dancer and mage cavalier respectively) and making you loose points, then give Ayra something on her C slot since she is missing something there. Also consider changing Quickened pulse on ayra since it's counted as a 100 sp skill and sp cost of the build is important, same with atk+3 on reinhardt if you end up keeping him.
Lastly, as the points needed to stay in each tier is incresing, consider changing your team in a team of character that can be found at 3* or 4* since merges weight heavily on your score

That team should be good to go from Tier 18 -> 19, except you need to use a bonus unit. Doubles your Arena score. I'd swap Ninian due to her low stat total for Fjorm. Olivia with merges is fine if you have to run a dancer. I use a 4 star+10 merge myself.

-Don't suffer any KOs since each one costs 10% of your battle's score. Actually, could have 1 KO and still make it.
-Fill out all skill slots, use high SP seals. Every 100 SP on a unit increases score a little. If it's a 240 SP seal like Panic Ploy 3, Drive Atk 2, Close Def 3, Quick Riposte and Heavy Blade then they're worth that much as a seal too. Don't use Atk+3 and the like since they're worth less, except if it makes a difference in a crucial KO. Upgrade seals if need be.
-Use a bonus unit at 5 star rarity. Fjorm is a viable unit with Quick Riposte 3 seal. Sharena and Anna are fine to promote to 5 star too as part of the Askr Trio that rotate being a bonus unit.
-Give your other units water blessings if you're somehow short on score. Counts as 2 merges each for Arena scoring. Of course, only works when you use Fjorm during a Water week such as now.

To start with, you probably want to drop Reinhardt long-term from your Arena team as he's one of the lowest scoring units in Arena. The longer you rely on him and invest in him, the harder you're going to make it on yourself to move up in tiers.

Simple ways to increase Arena score:

1) Always use a 5 star bonus unit or 4 star bonus unit that has at least 5 merges.

2) Use only high-scoring seals like Quick Rispote 3, Close Defense, Distant Defense, Smoke, Drive seals, etc.

3) Have at least one high-scoring unit to build your Arena score up (typically this is armor, but melee infantry (especially manaketes) and melee flying units can work in this role).

4) Use dual rally skills and Aether or Galeforce to significantly boost Arena score.

5) Long-term planning: replace any 5* exclusive units with easier to obtain units appearing at least at the 4* level. For example, Soleil is typically preferred over Ayra for most players as Soleil is much easier to obtain and is a fairly identical unit. You're fine with Ayra for now though.

6) Finally, don't use dancers. They score very poorly and will quickly cap your ability to move up in tiers. You can probably stay in 19 with one, but 20 outside of using PA Azura/NY Azura +10 is very difficult.