It needs to be buffed - but "buffed" does not automatically mean hardcore only.
Unbuffed or just slightly buffed means faceroll - and that means no long-term fun the whole community.
Currently, its hard to properly test and work on a good balancing, as different factors interfere on the PTR.
1) Naxx is totally bugged.
As long as this is the case, Bosses & Mobs dont do what they are expected to do, so the quality of testing is low.
Solution: Fix the known and reported issues: https://bug.tauriwow.com/index.php?do=details&task_id=23980&string=nax&type=&sev=&pri=&due=&reported=&cat=&status=open&percent=&opened=&dev=&closed=&duedatefrom=&duedateto=&changedfrom=&changedto=&openedfrom=&openedto=&closedfrom=&closedto=&search_name= 2) No appropriate gear.
Doing Naxx on PTR means going there with T7 BiS Gear. Therefore the content seems easier than it will be.
Solution: Add vendors on PTR for loot of each Dungeon. One Vendor per Dungeon Loot.
3) Unfinished balancing
Further balancing is required.
Solution: This needs time for tests, data analytics and re-tuning. My proposal to start with: Multiply Boss/Mob HP and DMG output by 2.