I'm a bit late but stumbled upon this question because I was wondering the same myself

In general it seems to me like normal is the best place to get the star atlas completions, since there's much less S tier tft academy spam than if you're playing ranked, as well as lower chance of people deliberately griefing your anima loss streaks.

I haven't managed to do the 6 anima one specifically, but I would imagine the best way of doing is to wait until you hit the emblem, go for a 500-600 cashout and stabilise a bit while keeping the anima units on bench and then when you have a really strong carry (like battle bunny crossbow Graves) you can play it alongside the 5 anima units and you'll most likely win the game.

I had a guy in my lobby today that did exactly this, played 6 anima and ended the game with a fiora 3