Decided to post this here since this community isn’t very active :(.

I had a blast with Swarm overall. It’s an excellent game mode.

I cleared Aatrox in the order Riven > the rest in order that they appear in the selector. I attempted it with full upgrades + lv 10-20 anima power (don’t remember the exact number) with the reroll behind "kill 40 elite monsters) still locked. I’m fairly certain you can attempt it as soon as you have full upgrades.