As long as you stick with raising skeletons/skeleton mastery early on with 1 point into Amplify damage, then points into Corpse Explosion, you’ll skate through normal/nightmare and early hell.
The only time you’ll have trouble is with normal Diablo. When your skellies die, run away and TP out to go somewhere else to raise your skelly army, come back. He’ll eventually die. I had to do this 3 or 4 times. Depends on how often he casts lightning hose, which murders skellies. My skellies were able to tank his lightning hose in Nightmare.
Don’t bother with mages until high level. I just 1 pointed mages and revives. (I’m using Iron golem, so I had to one point the mages)
Warning about Iron golem: There’s a bug that will cause him to be targetable when he gets stuck, your skellies will kill him! So if you plan on using Iron Golem, don’t use any expensive items to make him until they get that bug fixed!
Using clay golem + decrepify + cold damage source will render Baal an easy kill. He wont teleport, and he can rarely attack because he’s so slow he’s stuck in hit recovery. 
Be on the lookout for the Arm of King Leoric. 