This Arena threshold...

Merges are the name of the game in tier 20.

Maybe you're not doing Arena enough times? I don't know what people have to do to *stay* in Tier 20 (I don't even try to go to Tier 20 most of the time), so I can't really say.

Merges maybe? I manage to stay in tier20 with a merged team, with a Nino+10 Eirika+8 Nowi+8, all Nowi's and Eirika skills and seal cost 360 and eirika has DC, all of them have Aether and Eirika has double rally. Adding the bonus unit i try to get at least 726 per battle

Yeah, I just move between Arena 18 and 19, but as far as I know, Arena 20 requires heavy merges, a bonus unit, and costly skills. So it sounds like you have the costly skills, so just make sure you have a bonus unit on your offense team and merge 5* units to +5 or more.

First and foremost the most important thing is merges. BST, SP cost, everything else is secondary to merges; I use a healer and a dancer and consistently stay in tier 20.

Get yourself a +10 5* unit that you can build cheaply, or at least get multiple merges on your core team if you don't have that many of one unit as it's much faster at maximizing arena score compared to other factors.

Using an Arena score calculator is probably the best way to see what changes will actually help you increase your arena score. Keeping in mind that:

-BST is calculated without their weapons equipped and without the increased stats from merges, skills or seals. You can easily find each heroes BST on the Heroes page of this site. BST is also rounded down to the closest interval of 5, so there's no difference in scoring between a unit with 159 BST compared to 155.

-Sp totals are also rounded down, Meaning if the total sp of a build is 2170, it will not be worth more arena points then a build with a total cost of 2110 sp (sp cost being calculated with original sp cost and not the sp cost once inherited to another unit).

-Rarity plays a big role, getting a arena core of 3 5*+10s is going to be needed as the arena seasons progress. You can still use 4*+10s as bonus units since they might still be worth more points then an un-merged 5*.

-Although I'm not 100% sure what the Tier 20 cut off currently stands, I'd probably guess its around 722 since I stay in tier 20 fairly comfortably with scores in the 726 range.

I don’t come on here often, but I talk to several heavy hitters that have cemented their place in T20, so here’s what they always say:

-Merge at 5*, not at 4*. Higher scores are critical as the divide between 19.5 and 20 gets wider.
-high SP cost skills, which you are using, are also critical, so good job there.
-Seasons can make a MASSIVE SHIFT in the points required to stay T20. Last week, the cutoff was around 3634, which means you need around 726-728 per fight on average to stay. Water seasons and earth seasons, solely due to the freebies given out for Fjorm and L.Ike (and people getting more copies) make those seasons the hardest to stay T20 during unless you have a team dedicated to earth or a team dedicated to water, and even then it may benefit you to make multiple teams just to take advantage of the blessings if you possess the legendary hero for the respective seasons.
-BST bins are critical. 155-159 is a good start, but 160-164 is better, and common armors in the 165-169 range with a respectable merge count at 5* and good value on total SP are the easiest way to rack up a good baseline for scoring.

The divide each week has been estimated to grow by about 4-6 points at the minimum, not including the seasons (water season might break 730 per fight very soon as the bottom line to stay T20). But if you go through your barracks and find a good project or two to max out, you could become T20 within a couple months if you stick to a plan. If you’re late to the game though (I’m in the same boat, my scores go up, but so does the divide), it will become harder to stick when there are whales who are forcing players out right now, which is especially true for AA.

I got 726 x 5 in my run (3630) last week and it wasn’t enough, i just barely missed out by like 100 ranks. I got a +10 catria, +9 fae and +6 flyzura and a bonus unit, all with high sp skills. Considering that only the top 4000-5000 stay in t20 and that this is a gacha game, you shouldnt feel bad about missing that 1000 feathers. Just send home some 4 stars. But if you still want to try and stay in, i reccomend making 3 high merge units and using them with the bonus unit; it’s your best chance. If you get lucky with bonus units, you can get a pretty high score; can’t confirm yet but i think this can keep me in t20 next week.

Short answer is the dancer has to go. I'm actually surprised you score as highly as you do with one.

Also, 4*+10 is okay for your bonus unit but you really want to start 5* merging your arena core.