I just started playing necro. The leaderboards for him don’t look promising. I can barely find any builds. The meta build seems to revolve around no set and full ancient/ primal ancient. Which are quite hard to farm. Just looking at the available set bonuses from the necro legendaries, I don’t see much potential.
What are you even supposed to do with this class in the endgame?
It’s main use is for something called “Rat Runs”.
Still the best XP farming Meta in game.
Necros aren’t weak, but they’re for the most part damn squishy.
The LoD Mage build revolves around the legendary gem “Legacy of Dreams”. It is at it’s most powerful at max rank of 99 with ancients or primal ancients in every slot. The way that multipliers work though means that if the gem is rank 99 and zero ancient gear, it is roughly half as powerful as the fully ancuent build and is approximately 5 GR’s behind the fully ancient build in power. At Rank 50 the gem is roughly half as powerful as the fully ranked on so that is another 5 GR’s approximately. So a rank 50 Gem with no ancients should be about 10 GR’s behind a max rank fully ancient build (fully ancient gear has more main stat and can be augmented so it’s not so cut and dry about halifing the power). You should have little difficulty ranking gems to 50+ with the right gear and even rank 25 gems can get you to farming T10+ while farming for the right drops and upgrading spare gear to get yhem as ancient.
The necro is currently the second weakest class for gr pushing. With the buffs to crusader and barbs and then the nerf to wiz the necro is in a bit of an awkward spot in that it’s not any weaker than before (aside from thorns necro being gone, but who actually enjoyed playing that?). The necro won’t push the highest in the leaderboards, but you’re only competing against other necros. What necros are good for:
If you’re just starting the necro, I would highly recommend lord fluffy’s videos on YouTube for his blight spear build, lon singularly speed build, and blood and essence builds. These builds are all very fun to play. As a necro you just gotta embrace the squish. If that’s not your thing, then hey, the barb just got some dope new builds including a ww build that is very low key play style if that’s your thing 
Oh and I honestly wouldn’t worry about comparing leader board clears against different classes - the balance is all messed up right now, but it doesn’t really matter because each class only competes against itself.