Just did this encounter on 2nd play through:
Got caught talking to prisoners, but not before getting word of the escape attempt.
So I went to the far end of the walkway where there's only one guard and my Astarion assassin used wyvern poison to eliminate him. Looted 'em and stashed the body in a crate.
Strung out the party along the walkway and went turn based to time enemy movements. Wyll had Hunger of Hadar ready to slow and block LOS if need be.
Floating eye came along. Assassinated with arrow of construct slaying.
Wandering guard came along. Poison ganked. Body stashed in pile of rags.
Next Floating eye. Eldritch blasted.
Warden? Got a good dose of poison with critical hits. Party is optimized for initiative wherever possible. Didn't get an action. Third floating eyes dispatched.
Heck I'm on a roll. Any solitary Absolutists are going to get picked off and body stashed. Don't think I can one-round-kill pairs with my balanced party though. Probably need Lae'zel for Shadowheart to get DPR high enough to try.