Dying to Off screen enraged enemies In maps and Losing a boss Encounter Is just bad design

Died to A Rare mob on a hill that i could not see, With the enrage Modifier. The Mob was behind bramble and out of view. I have 70%+ on all Resistance's except Chaos which was 40% and am Playing mercenary. This map was a tier 2 with no mods.

With That being said, it jumped over then bramble still barley in my line of sight when the attack landed and one shot me. There was no counter play or response, I did not roll because what i was was the mobs sword touch the upper edge of my screen (I guess the shockwave hit me full force or something).

My issues are the following.

1. The enemy's aggroed me from outside my line of sight.

2. the enemy's attack began and ended outside my line of sight.

3. I just started maps, I managed to get My resistances in order after Using a ton of orbs looking for solid rolls (140 Of various orbs each),

4. I managed to get minimum 70% on primary 3 and 40% of chaos. and he still one shot me because 37% armor DR with 40% evasion just means 40% evasion only. So that failed to trigger and he managed to one shot me due to enrage, and the ability to clip through walls.

5. My boss map with only 1 boss and 1 rare left is now in the Aether never to be finished. with no ability for me to counter play

6. most of the rewards were advanced gear which i already have to the highest mods i can get for defenses, So upgrading my gear is Not Feasible since higher stats then 35% on multiple pieces is not possible on my Lvl 73 character because I'm stuck with level 59 gear.

7. the boss mod on the zone is now gone due to things beyond my control, so i cant even try again with there Knowledge i just gained.

In short none of that experience was Fun, it did not add anything to the gameplay for me in fact now I'm wondering if i should even do maps, that's not the first instance of being one shot. Before the full resist gear I have, I had a chest piece with 660 Evasion which brought me to 57%, it was fine but the minions who leave pools of Blood + Multiple Item drops means Invisible Pools,

So not only do you have to have a High DPS build, Play well and have decent defense, you also aren't allowed to be excited and immediately collect drops you just won hard fought.

Its just ill intentioned game design, My fun is slowly dwindling away because of all of these mechanics that prey on the high moments of difficult gameplay, like after death AOE.

TLDR: I died to an off screen enemy, I have Very high resistance's and just started maps so my gear is stuck at 59-65 even though I'm above 70. this is the 3rd time I die to Mechanics that prey on players fun, such as Loot blocking after-death AOES from being seen, Enemies that jump from off screen and One shot you, There being literally so many Mobs on screen even dodge roll cant find a path to move with and lastly Mobs that eat mana in a Huge range Around them that also have 40% increased move speed, Assuring that you don't have any ability to Fight back other then kite around the map.