valks only get that retreat from battle if killed with lecia (black valk, new event unit) is deployed.

also, berna is nice post-AW, but chances are you'd get more out of AW'ing other units e.g. clissa for her true dmg + additional stats against whatever she is dueling. and you need to remember that's only post-AW for berna to really be of any use. assassination is also not very critical late-game since chances are you can just burst dps down everything, and berna wouldn't be a very good choice for anything that can't be burst-dps downed. Majin Advents are a good example of the really good units that everybody would use, when confronted with large numbers of crazy strong mobs.

in any case, berna is just good because she's a black, but her class and stats doesnt help herself nor her class any.