How to get in teir 21 arena

By having no less than three 5+10 units with your bonus unit and being good at resource conservation and investment.

Merge up armors if you have the fodder, dragons if you don't. Also merge a buffer unit like Marth or Eirika to buff your bonus units, they need to get all of the kills.
Then, wait for a time when an OP hero shows up (ex. Legendary Tiki), that you can use to easily get 4 kills every match. Summon said hero with a bit of luck, use your +10 merged dragons/armors & 1 +10 buffer, and get a score of 734-736 +12 or higher.

I got in on the secondary Legendary Tiki week with a +10 Eirika, +9 Nowi & +9 Fae.
Also use as many dual rallies & 240 SP skills as you can.

To get into tier 21, you need to have three 5*+10 units with high-SP builds (tier 3 specials, dual rallies, etc.). You also want your bonus unit to have a premium build. The 4 Askr units will suffice.

I'm not completely sure what the cutoff is but I've found that getting 3-4 kills with the bonus unit when playing against teams that are 734+ is sufficient.

You will not be able to stay in tier 21 unless you have multiple 170+ BST units and/or heavily merged bonus units.