In Path of Exile, I am playing a necromancer but find myself dying to one shots. As a necro, I have a life pool of 6.5k with 1k Es. However armour and evasion is Low. So I summed up some tips on how to prevent one shots in game, I hope it will help you, and don't forget poe trade currency can make things easier.
Dropping life nodes for armour is the exact opposite of what you want. Armour is crap for preventing oneshots unless you have five digits of it, and still only protects against physical damage. Life is by far the best way to prevent them.
As a summoner you can be quite tanky, but it doesn't sound like your build is. It's not squishy either, but 6.5k life and some other random stuff is kinda average. Enough to absorb some stuff that gets through, nowhere near enough to facetank. That's perfectly fine.
Your best defense though is just not to get hit. If you're dying to tentacles or icicles in the elder fight, you've made a mistake, pure and simple. You have to shield charge more, circle strafe more, just generally get better at dodging. There are almost no builds these days that don't rely on this to a large degree in the hardest fights.
Elder have 2sec animations, he rotate 4skills so after u see first attack and dodge it, be prepare to next attack, and next after next, so its relly crap died on this scripted.
If you still not know his skill set watch any kill video with 0.5speed, but skeletons, so shity for new players, if u new play solar guards, after u feel dps on bosses learn how swap gmp, after u feel sick solo dps again respec to 2x2 setup(2solar+2tw).
But skelie differend u lost movementskill+block for tradeoff paper dmg, play first time summoner destrpy uberatziri/shaper/redelder with 5lsolars and deathless (ofc lost spectres on uatziri bcs forgot about covc minions out and they died on tripleblast).
When you encounter some problems that you don't understand in the game, please don't panic, you can log in our official website on https://www.u4gm.com/path-of-exile, there are some guides and news related to the Path of Exile for beginners.