Might I suggest…
A quick look at your seasonal Necro…
It’s unsurprising you’re struggling, as he doesn’t have a full armour set (the 2, 4 and 6-piece bonuses give large damage / mitigation bonuses). The alternative to which is the Legacy of Dreams gem, which gives bonuses to damage / mitigation based on how many legendaries you have equipped, as long as you don’t have any set item bonuses.
There’s a section in the guide I linked which covers builds for “fresh 70s” and how to progress into full armour builds.
Oh, and in case you’re unaware, in-game, press Shift+J to look at the Season Journey. Completing Chapters II, III and IV will reward you with a Haedrig’s Gift. The first two gifts contain 2 pieces of a class-specific armour set (determined by the class that opens them), the third contains the remainder of the set (which can be 2 or 3 pieces, depending on whether the set has 6 or 7 pieces available). As soon as you get the 2 or 4 piece bonuses, that’ll be a major damage improvement. These gifts arrive in the in-game email system - you’ll see a little green envelope at the bottom left of screen for this.