This one is a bit tricky unless you have more than one great attack team attack between which toons aren’t shared. If you do, then you just need tickets and keep entering to place well. If you don’t, that’s when a lot of factors come into play.
This one should be easier then raids since you can see your opponents teams and pick who to fight. This arena is also listed as endurance, so the total from all your entries is taken as your score.
This arena wears out three of your toons at the end of each entry (entry not attack) and you can only choose to save one of them, hence my point above on having multiple teams. With enough tickets and combinations of toons, you will place well automatically. But to the most of us who don’t, we will need to make that first free entry matter, especially if you make do with one team.
I would suggest following a different strategy in this arena as you can’t see the leaderboard. Since there are no restrictions in this mode and everyone is using their toons, the best way to place high would be to pick 4 opponents from the pool that you are sure you will beat and attack them in ascending order. This will most likely place you high as there is a high probability of the majority of people losing toons in battle and getting reduced points or even just having one botched entry since their team is now incomplete.
The same should apply to this weekend’s champions arena except that the entries are limited by Champions Tickets which are currently only available as both milestone and placement rewards in the arena, throughout the week. This tickets also expire once then champions arena closes, so be sure to use them all even if you score a bare minimum.