General rules of survival:
1: Max con, always.
I'd rethink that.
While I wouldn't neglect constitution I wouldn't necessarily max it on a 28 point build. The point buy costs rise. When you distribute points you can at least put 6 points into that stat, which will give you a 14 with most races, 12 with elves or 16 with dwarves or warforged. Another 4 points will raise your constitution by only 2 constitution points. And then it gets more expensive. What do you get for that? At lvl 30 thats just 30 hit points for every 2 points of constituion beyond 10.
In the meantime you get more benefits from some low level ehancemnents and enhancement trees, false life items and vitality on armour or shields. Later on you might get set pieces with set bonuses which increase hit points. Some high level enhancements and destinies will raise your hit points by around 5%-25% respectively.
I can only recommend to not start below 14 or 16 constitution depending on class and race.
There are spells like false life and aid which will give you additional temporary hit points.
2: Invest in other defensive stats. Always try to have a good saving throw resistance item. Aim for fortification of enemy CR + 100 (so if your fighting CR 15 enemies, you want at least 115 fortification). PRR and MRR where you can. Blur spell or clicky or item.
You need to know when blur and diplacement is useful. I'd rather check the opponents first. If they have true sight or similar, then it's a waste of sp.
It might be a bit difficult to get high PRR and MRR depending on class and available gear/ armor. On higher levels and with crafting it might be easier.
The same goes for AC. Certain classes have an advantage (see enhancment trees). With heavy armour you will get lower dodge and vice versa.
Fortification is really helpful.But only protects you from critical hits.
Saves is also not that easy especially if you've dumped the corresponding stat. gear can help, though.
Regarding elemental damage it's useful to renew elemental resists and protections. Especially the latter when it's eaten up. The same goes for stoneskin and similar spells that protect from physical damage.
Depending on class and race choices you will have limited options besides gear.
3: Avoidance. they can't hurt you if they can't hit you. Whether by staying at range, or by utilizing effective crowd control.
Over the years CC spells got nerfed, especially regarding duration and thus usefulness. Now it gives you a rather small window of opportunity to act. With a full party it's enough time. Alone, depends on your build and DPS.
Avoidance is easier said than done.
4: Knowledge. This one is actually huge. Players who know the quests well, know their build well, and know the game well, can do amazingly well even on sub-bar builds.
This will help a lot indeed. On subpar builds it will still be difficult.
5: Know your limits. It is OK to go slow. It is OK to pull smaller groups of enemies and deal with them in smaller chunks. It is OK to run on a lower difficulty (especially this season if you're not going for leaderboards), and it is OK to skip some quests. There are specific quests that are deadlier than others, contain instakill mechanics, have enemies that are uniquelly devestating... it is OK to skip some/most/all of them.
It is very possible to do well in hardcore solo. There are onyl a few quests that actually require more than one person. A group CAN help cover your own weaknessess though, so grouping does help as well, provided you get in a good group of knowledgeable people who co-operate and want everyone to succeed. Sadly, there are some individuals who do not do this, either by going on auto-pilot and ignoring everyone else as they do their thing (inadvertently getting others killed), and there are an even smaller group of players who seem to actively try to get party members killed (some will even laugh abut it, so yes, they do exist).
