Well first off this mod is different from classic D2LoD, better in many ways i would say, and filled with new stuff you need to learn if you want to progress. Normal difficulty can be completed by doing anything with any class, Nightmare will require you to improve dmg/def/resist/etc and Hell is where you can start doing Ubers, which is the endgame and where you will spend more time that in the combined story mode of all difficulties, which has become trivial in comparison to the challenges/rewards found in Ubers.
Now since Necromancer had a rework with this last patch, and Summoner Necro is quite strong (very good in most ubers with just a few more tedious exceptions where you could just respec) i suggest you try it, i played one myself and sometimes it felt like cheating, maybe cause i mostly play melee characters, and this summoner is very safe behind his minions so i could focus more on dmg rather than split dmg/def/leech as with my melee characters. I think from behind the safety of your summons you can see which boss does what and learn what to avoid for the future playthroughs. Also plenty of available gear choices you can easily create, good Tiered Uniques, Runewords and even Honorifics with which you can get mid way and even further into Ubers, both Sacred sets for summoners are good if you complete them and if not you can craft some jewels and Shrine craft gear to compensate, make some XisRunewords for gloves/boots and anyway the idea is all these are obtained without too much farming, and of course you will eventually find some Sacred Uniques too (cause these are the only ones not craftable). Just prioritize minion dmg, then minion life and maybe a little bit of minion resist if needed, you have MOs that work on all items for these, minion AR% you get just from gear and i think 200-250% may be enough, minion dmg will get over 1500% minion life close to 500% and minion resists at 75% are fine but you will get more anyway.
Before endgame and all the +max skill charms you will have more skill points than you can spend but i'll say what i did anyway, maxed the Summoner passive and Hawks (suicide summons), as much as i could to get to the next +1 extra minion into Skeletons and Knights (both have good dmg and are resilient), as much as you can spare into Embalming and 1 point into other support skill tree skills, and as ultimate skill maxed Rathma's chosen. Golem i summon from random rare items on ground and he is quite tanky, and fastest one i think, good target for Bane. So generally use Dark power oSkill from gear to buff, Bane for defuff when you reach bosses, spam Hawks and kite projectiles if any of them get passed your 20+ summons. If you have mana issues (you may at first) use %Mana regen MOs on rings/ammy, just on 2 of them, then even just 1 as you get more charms with energy/mana. I ended up puting all attributes into VIT, but earlier on i had them into STR/DEX for gear and NRG for mana, because you can free respec once per difficulty or with Belladona Elixirs that can drop anywhere, but it's a rare drop. And as merc i used Shapeshifter, his Bloodlust buffs dmg, doesn't die often in farming levels and well mostly to spells that are meant to 1shot things.
Lastly i also recommend skimming the docs https://docs.median-xl.com/
so you know where to look for Cube recipes or tips on Ubers as you need them. And of course from tools section https://www.median-xl.com/tools.php
always use D2stats for more detailed info on minions and just drop notifier and when you may need it SpeedCalc for hit recovery and cast speeds.