TL;DR - Enemy thrown grenades too OP, needing either fair HUD indication, re-balancing, or outright removal
After playing Warframe for several years nearly since its conception, I've noticed one annoying, consistent thing in gameplay: faint, fast, high pitched (sometimes inaudible) beeps followed by a *boom* and instant death.
While obviously not a universally common complaint (maybe it's just that obscure!), I've run into several accounts (spanning across the years!) of players complaining about ambiguous or unclear one-shot deaths, usually in high level missions such as Arbitrations, Elite Sanctuary Onslaught, but primarily against any enemy with a pocket full of inexplicably powerful thrown grenades.
This is my first post offering feedback, considering that, similar to others, upon checking out the game's 'death log' (appdata/local/warframe/EE.txt), instant death-by-grenade is shown by ...
14:07:04 - <OcelotEx> took 2,774 damage at 1,225 health from a level 99 CORRUPTED LANCER using a NpcOrokinThrowGrenadeWeapon
...Which I've found a handful of other players taking note of. One example:
'Resisting damage' or 'the proper build' is no longer the point in the case of these grenades, the point is the damage is unnecessarily hard to avoid entirely.
Consider a tough enemy like a Corpus Nullifier or Grineer Bombard. While more or less capable of such damage, upon seeing either of these enemies at high level, a player can determine how to react. The same can even be said about Sapping Ospreys (consider how much damage they deal.) In the case of this obscure 'enemy thrown grenade' mechanic, the only chance a player has to react is the hardly distinct, quick beeping noise, and many times the beeping isn't even noticeable. Not only do these grenades 'insta-delete' non-tank Warframes at high level, but the indication that your 1,225 health is about to be zero simply isn't enough to react to, or needs updating/balancing. I'm actually unsure whether this kind of damage is intended, or some kind of scaling bug.
14:07:04 - <ShedDecoyAvatar7145> took 7,721 damage at 107 health from a level 99 CORRUPTED LANCER using a NpcOrokinThrowGrenadeWeapon
The simplest solution I can foresee is to have a HUD indicator of enemy grenades (like close to every modern video game involving thrown grenades), or for the sake of lessening confusion, an in-game death log similar to Conclave/Index/Rathuum. The latter would serve to nullify the broader "wtf killed me?" dilemma, while the former would directly address the grenade issue. Although, I don't know of anybody that would mind seeing them removed altogether.