Alright there's a lot to break down here.
You shouldn't be dying in archon form and if you have the right rift, dying is less common outside it. Have to learn how to take advantage of the most groups of mobs possible to proc Halo of Aryse and Ancient Parthan Defenders. If you don't have a large enough group, you're screwed. Even then, the procs only last about 7 seconds on a group of mobs, so you have to use that time to get back into Archon as fast as possible.
To further help survivability, you should be using Esoteric and Orb of Infinite Depth. If you're using those, keep in mind that you probably have low gem levels and no enchanted ancient gear. Expecting to do GR80+ without that is fairly unrealistic.
All this being said, this build is very much a high skill playing build and it does benefit the most from high density rifts (so there is some degree of fishing). It's not going to be for everyone. I still suck at it .
You get to a point with the DMO build where your bubble is up almost all the time, so placement is never an issue.
Ranged mobs aren't much of an issue, because you don't need to target specific things. You use Archon Teleport to go to as many groups of mobs as possible and can use the beam to aggro mobs. Some people actually use the beam more than the smash, so it's perfectly viable even against ranged mobs.I really regret picking wizard this season, firebird archon is such unfun gameplay imo. I cleared 70 with it but don't know about 75, it just feels like 3/4 the rifts you might as well just restart the game, because the spec is absolute garbage at dealing with ranged mobs. Damage is stupidly high, so as long as you don't die, completing a rift isn't hard, but if it's filled with stupid ranged mobs, you basically die all the time, even in archon form, and that's with 2 cheat deaths, because you kinda have like no mitigation or it's conditional. It's just very meh to play.
You shouldn't be dying in archon form and if you have the right rift, dying is less common outside it. Have to learn how to take advantage of the most groups of mobs possible to proc Halo of Aryse and Ancient Parthan Defenders. If you don't have a large enough group, you're screwed. Even then, the procs only last about 7 seconds on a group of mobs, so you have to use that time to get back into Archon as fast as possible.
To further help survivability, you should be using Esoteric and Orb of Infinite Depth. If you're using those, keep in mind that you probably have low gem levels and no enchanted ancient gear. Expecting to do GR80+ without that is fairly unrealistic.
All this being said, this build is very much a high skill playing build and it does benefit the most from high density rifts (so there is some degree of fishing). It's not going to be for everyone. I still suck at it .
There are some very powerful DMO builds. Look up DMO arcane orbit from season 5 on diablofans: http://www.diablofans.com/builds/70759-s5-gr90-solo-arcane-orb.The DMO starter set was also a pain in the ass, glad I got Tal Rasha asap for farming. Just changing my build to Tal Rasha using the same gear in other slots I jumped from T6 to T9. I mean I could have done T9 on DMO, it was just way way slower, even T6 was taking 4+mins per rifts. Again damage wasn't an issue, it's just the stupid slow time mechanics and how the set literally does nothing unless you have slow time on the mobs you're attacking and are standing in slow time also. Having such restrictive mechanics makes you really notice how many mobs have stupid knockbacks or run away from you. Meanwhile with Tal Rasha, you just kinda click stuff to die, like most other sets in the game. Hell once you have the whole flashfire stuff, you don't even click stuff, you just run through mobs and click items that look shiny.
You get to a point with the DMO build where your bubble is up almost all the time, so placement is never an issue.