Optimizing Arena Score

A higher rating in-game doesn't mean higher score. Fury gives +12 to the rating but does not give more score than other skills like brazens or stances. You need to look at merges and bst without their skills. Armored units have the highest bst in the game. Take a look at this list: https://fireemblem.gamepress.gg/heroes and sort by ''total''. (you'll see Winter Lissa and Hardin have the highest BST).

Edit: forgot to add that non-inheritable weapons and skills also give more score but seem to give lower "rating".

To what rating do you refer?

All that matters at this point is choosing high merged units and giving them unnecessary skills that require heavy SP investment.

Trying to get past tier 19.5 isn't fun anymore.
At least not for me.

...I'm still trying though

Pop your units into this https://www.arcticsilverfox.com/score_calc/

I only bothered looking at what my units were scoring a month ago and now I'm 19.5, nearly 20 instead of 18.5. Merges and high SP skills like dual rallies, Aether, Swift Sparrow, Renewal, Drives etc are what you need.

The way Arena Scoring works really is a terrible system.

BST and Ability SP Cost are terrible ways to determine a score, as it ultimately creates a stale environment where everyone runs very similar setups all the time.

Here are the conventional answers:
- max out merges and the seals you're using
- if you're using a low BST unit, consider replacing this unit at some point. BST-based scoring is stupid but it's the current system.
- use high-SP skills (ex: dual rallies over positioning skills, drives rather than spurs unless they're dual spurs; breath skills or close/distant def instead of fury or the infamous skill of preference for SP weight, distant/close counter), but check this with the calculator because you might need to swap out several skills before the score of a character increases. From me especially, be careful when doing this because if you screw up your build, it's not worth it. A bit of compromise or looking for alternative ways to do things is fine, but being able to actually fight is the most important thing in the end.
- use builds which make good use of aether or galeforce. Same idea.