This problem absolutely is black and white. If you're standing in a telegraphed area and the boss casts the "YOU DIE" spell, and you're just standing still, you absolutely expect to die the moment the castbar ends in any MMORPG. Unless other MMORPG have manaaged to figure out how to teleport data, the client absolutely is the one making the call here. Hacking is way overblown as a concern. If you get hit by the 'YOU DIE" spell, there's very little the client can lie to survive that. You can claim you didn't get hit by it, but the client still sends your current position and the server can see you're lying. You can change your position too but that's teleporting which is very easily caught. You can lie about the damage you took from but the server knows you're supposed to take a million damage from YOU DIE too. You can lie about what CDs you used to survive it, but the server also knows when those CDS are up.

The best case scenario is you might be able to lie about using a CD you forgot to use (but is still available), but you could just learn to use your CDS too.