My biggest advice for Hardcore players is do main story ASAP, when your 2 levels lower or at level.
This isn’t Diablo2 where you can overpower bosses further level you are. Level 50 is going to fight a lvl 50 boss, leveling becomes super slow in World I II, after 50, so your not getting any benefits from trying to outlevel the bosses. They just get harder and harder. So do them early, when you can almost use any skill/build and beat them with ease.
When you get to 50 those bosses scale to 50, so if you all sudden are bad, well the bosses are going to be even worse.
Level 20 30s, as long as your 2 levels away from what it is expected level area, you should not have issue with the main story bosses.
btw I did world boss Avarice, got 1 shot by him at 51. Meanwhile there are level 36s able to take a beating from him not much issue.
Feel like hell level is 50-75. If you made it past 75 you should be golden.