Well I just hit level 70 on my Sorcerer and decided to give it a shot.
Skills are:
Spark
Chain lightning
Flame shield
Lightning spear
Frozen Orb
Unstable Currents
So I get into his room and he appears, I start hitting him with everything I got, and within 35 seconds he drops dead.
Last season he was much harder I felt.
Or has the Seneschal just added more dos and protection than we thought?
Seneschal adds a lot the further you go. Ignore people who say it is weak they have not played past level 20.
Don’t believe me take the Governing Stones out (essentially turning it off) and do a dungeon you just did.
I did this by mistake as I was tweaking my build for more power, and having him off makes it easier to see my damage. Then someone asked me to powerlevel them. I forgot to turn it back on, the capstones were no issue as they are much lower…but the first dungeon I did in WT4 was shockingly harder than it needed to be, until I noticed I had forgetting to turn him back on. 
Also if you are lvl70 Elias should be no issue.
it all depends on your class for some its an easy fight for others its hard.
I don’t know about you but I’m a much better player now than I was when I got to Elias last season so just having had more practice in the game was a big help. I went from dodging explosions in AoZ 20 to Elias a few days later so it certainly didn’t feel as hard and I killed him at 56 when dodging his skills is crucial.