1) It's always hard to tell what happens in a casual game and what makes people target who they do target, but a relatively safe bet is that you're overexposing yourself too much. This is a common mistake newer players do, and it is exacerbated when you play melee because you'll almost always be taking damage by one source or another since you're the closest and easiest target to hit for the enemy. Playing melee needs you to learn how to dive in, do your damage, soak up enemy damage, and zoom out once you're out of resources. It's true for all classes, but more so for melees.
2) The mechanics on how the server works decide how everything resolves. It's way more important to understand how it clicks in pvp than pve because not only it's more convoluted than pve due to crowd control, but it's also more prevalent between netting a hit or not:
- Pressing a button immediately registers an ability server side. As soon as the ability is registered, it will snapshot EVERYTHING on that time stamp, and any crowd control linked to it is also applied immediately (this is why all crowd control is extremely snappy and happens as soon as you press it).
- Damage resolution waits for the skill animation to go off with an arbitrary delay that's unique to every action you use (a bit like in pve, but blown out of proportions, especially on LBs where it can take several seconds). Even if a target guards in between before your damage resolution happens, the damage will get through because the skill was snapshot before that guard. However, since heals are instant, recover will heal before your damage resolution, which makes it distinct from guard in that regard.
How does this translate for Zantetsuken?
- If you pop the LB before the purple debuff shows up on somebody you hit, even if the debuff shows up before the damage resolution, you'll not get the oneshot because the ability snapshot the skill before the debuff was up.
- If you pop the LB after the purple debuff appears on somebody, it will snapshot the skill for the debuff and even if the debuff has faded off when the damage resolution happens, you'll get the oneshot. What matters is popping the LB during the purple debuff, so as people already said above just make sure to see the debuff on targets before using the LB (and make sure to hit the correct ones).
Sidenote: do not die before damage resolution, as dying negates damage resolution, snapshot or not.
Last sidenote on Zantetsuken: it deals 100% of the target's HP, which means a target with enough HP and a shield will probably survive, albeit at low health.