You should be able to stay in tier20, to permanently stay in tier20 you should make an arena core with only 5*+10, just think of an investement which will slowly return to you
Hopefully you stay. The last time that Fjrom bonus and Water season were both active back in early June, the required score to stay was exactly 734 per match.
Well, did you end up staying? I am often shocked by how far I fall in the final minutes of anything in this game. In the last tempest trials I went from rank 3,000 to rank like 10,001 in the last 2 hours.