The way you fight bosses (or single targets) is exactly the same, only you don’t benefit from area damage and you cannot generate essence that fast. To compensate for that you kinda want to time your rotation of COE. When it rotates to poison damage, that’s your 4 second window to go all in, stun the boss and start throwing as many bone spears as you can. Then for the next seconds you can start hitting scythe to generate your essence so you can have plenty of it for the next poison rotation. There are elites that have the juggernaut affix that means they are immune to crowd control and make your bane of the trapped and krysbin sentence worthless. If you struggle against them just skip them.
You get around 83% damage reduction just from your set. Yes this is a tanky build. 50% is reduced just from the start, then the other 50% that passes is split between 3 targets. You, 1st simulacrum , 2nd simmulactum. That means (2/3) * 0.5 = 0.33 (33%) is taken away. Hence, 50% +33% it adds up to 83% dmg reduction. Now lets factor the next item. You alreay have 83% dmg reduction, that means 17% hits you. If you take let’s say for example dayntee’s binding which is 50%, then that 50% of that 17% that hits you is also reduced. 0.5 * 0.17= 0.085= 8.5%. So in total you have 83% from the set plus 8.5% from the belt, equals 91.5%. For the next item in line you should multiply that remaining 8.5% with the dmg reduction percentage of that item. It goes on and on like that. This is a very tanky build, you get a lot of damage mitigation layers. As you said it, Razeths volition, dayntee’s binding and bone shield added on top of the already tanky set. From that point all you can do to increase your toughness is to find better gear and increase your paragon.
I saw in your profile that your gear are legendary and you are sitting around 1k paragon. If you start finding ancient pieces, assuming of course that they rolled with the desired stats, not only they provide higher intelligence, vitality, armor, but more importantly you can use the last recipe in the cube to augment them even further. A good milestone is to level up gems up to lvl100 and augment to your gear. This means that if you find 13 ancient items, 13 * 100*5 = 6500 intelligence. Trust me, if you add this number of main stat to a low paragon character it will make a significant difference. Both damage - wise and toughness - wise. Remember, intelligence will increase your toughness through all resistances.
Another thing that i like to do is use the final service passive. Every class has a similar passive that protects you from death and it procs once every 60 seconds ( it cannot be further reduced with cooldown reduction). Change one of your offensive passives with final service and try it to see if this makes it easier. What you can also do is replace the gem in the helmet with an amethyst that gives you more life, altough i doubt that this will save you if get one shotted all the time.
To give you an idea, i am 1700 paragons and have augmented 7-8 of my gear with 120 gems. I believe i have around 17-18k intelligence. I can speed farm gr105 very fast, under 3 minutes with a good map and the only thing that can kill me is if i sit inside molten explosions and maybe another one or two annoying affixes. Although, i am currently playing my seasonal necro and i have access to the 4th cube slot which is another weapon to add more damage to the build. There are things that can hurt me, sure, but my damage is so high that i one-shot the entire screen with my bone spears so nothing can come close. In some cases, best offence is the best defence.
To sum up, you kinda want better gear (ancient with the correct stats) and higher gems (your gems are very low). Then start augmenting them. In the process, you will also be increasing your paragon.