There is no "react" to it, no audio, visual support will help you. There is no counter play, you just get 1 tapped by all means, Guard, full health, mitigations don't matter. I have only managed to dodge a samurai LB once, and that was because he was dumb enough to stay outside of my sage LB so i was invincible to his attacks.

Not sure if there is anything else that can ignore the 1 tap, Tank invuls I reckon.

Some humanape(samurai main) was trying to justify to just not hit him while his buff is up that provides the debuff on the attacker to get onetapped, not sure if he realised but the current PvP, at least CC, has a lot more aoes at hand. So you may not even realise that you are debuffed to get 1 tapped.