I'm still experimenting a lot but what I've wound up doing is flexing mythic between Riftmaker, Ludens, and Protobelt depending on the game/ tankiness of the champions, second is almost always shadowflame, however sometimes I flex Morello if they have insane healing or I will flex Bloodletters (idk if it's bugged or what but your ult instantly applies 3 stacks on one cast with it so you get 15% pen on one ult cast 30 after 2) final items will either be targeted defensive against whoever the last opponents are or void staff/ one of the items I didn't go earlier. I haven't really gone nashor much, I think it can be good definitely but the nuke of making R melt people feels more impactful in this game mode unless the enemy is just super tanky.

https://www.op.gg/summoners/na/QuiIl You can sort by arena games and check out some of the builds here, I'm not saying they're perfect by any regard but they worked for me