No AOSBs. Without giving a full breakdown here, Ashe and Shan both with USB/dive, and Shan has LMR, but those two are my main damage dealers. Using Prompto for lightning chain + spark offering + dive to stack up the imperil, and OK using Voltech to push out some more damage, which he does quite well and can cap damage on the tail end of the chain. Prompto has entrust in his 2nd slot to give a small boost to Ashe to get her going a bit faster. Ashe caps out damage pretty quickly into the chain using Chain Thundaga (don't have rubies for Chain Thundaja), and Shan runs her witch spells and starts doing more damage than another Chain Thundaga pretty quickly. Between Elarra using USB and ACM, and OK's initial mUSB and recast around 20 seconds, everyone has quickcast for the majority of turns.
I think the problem is, as you noted, I don't have a "finisher" SB, so when I get to phase 3 I need to have Elarra ready to set Ashe/Shan up with last stand and see if I can't push it down the final bit before he takes them out.
As a side note, I think I've been incredibly unlucky with Shan's LMR proc'ing while under her double cast USB, because I've not gotten a triple or quad cast, whatever it is