Defeating the celestials are are not part of HC achievments, but I agree. In order to be safe as Foehammer suggested - best to play Warlord or Warder. I found Pet Conjurer to be a good choice aswell as you just let your pets do the fighting 
For Warlord I recommend this one (used it myself for HC - easy to farm gear even while levelling):
About phys change - enemies have less phys damage now but your ways of getting phys res has been limited a lot on gear, devotions etc. That’s also why Warlord really shines here as a shield with “Durable” prefix (or some of the rare affixes) already gives lots of phys res
Nemesis spawn - Nemesis now spawn somewhere close to you if you are on nemesis rep and kill enemies of that faction