I don't know how you guys get to endless. I end up fighting a group of 4/40's or 5/50's with the damn regeneration from negative effects and I'm toast.
The trick is to keep your champ level a bit lower until fight 14. Fights 7-8 with an even distribution under 4k will match you with 4800-5500 opponents, with typically easy AI.
Fights 9-13 use a 2 max 3* with one unranked 3*. This is one tricky area, where I've lost my bid for endless a few times. You have to win the 2 matchups, so look for at least one good class matchup. Also, be careful of the random game crash. You may want to reset the game before you start this stretch. These fights aren't too bad, but occasionally you hit a tough AI match.
Fights 14-15 use your best team. I have 2 over 6k, so that's what I use. I match up with about 10k teams, but these are Thanos / Kang teams without the best masteries, and typically without the tough AI. I usually find two or 3 class advantage matchup in these.
From 15 - 20, you'll start the easy matchups, but you have to keep your PL total high. The blueprint says over 6k until 20, but I played with that some during the cornucopia this week. I stated with my best teams, then on match 17 I dropped down to about 5700, then 5400 and finally 5100.
What I think happens is that there is a gap that grows / matchup in your streak between the team you put up and the opponent. Once you hit the max gap, you get a Thanos/Kang team ( before match 15 ) and an easy match ( with tougher AI ) after 15. As your streak grows, the max PL of the team you use can drop and still get the favorable matchup.
What I used to calculate the target totals was take the blueprint numbers ( 6k at 16, 4500 at 21 ), and linearly step down, so drop about 300 PL each match. It was only one time, but it gave me the easy matches from 16-20, as I hoped.
Full disclosure: I haven't hit endless this arena in two tries, Caught a stubborn L3 on match 15 in the first go, and slipped on an evade to storm's L2 in my second attempt at match 13. This isn't foolproof, and you have to be decent at the fighting aspect ( I'm *maybe* decent )