summoning necros had a hard time lvling in classic and all of LOD until 1.10 came out. since its been a few years since i played classic, all the advice i can give is try popular act 3 spots (trav, kurast) if not then i think it'll be better to advance into act 4, since the leveling is much faster there, and just hope your skeles dont die to quick.
If you want ... you can do a quick search for my nickname and you'll find in detail my experiences trying to finish Hell on Classic with patch 1.10 with a summoner/exploder (artillery necro).
It's easy up to Hell and then it gets almost impossible by Act4 ... mainly because you:
(1) don't have a merc to help you get that first corpse
(2) it's really frickin' hard to get that first corpse
(3) lack of +skills equipment and decent items etc in classic. Hello Tarnhelm! Hello Stone of Jordan! Hello Silks of the Victor!
... but I have it all summarized for the most part in my previous posts.
Even normal bugged the hell outta me.. finally I realized amp + CG to take the hits and if I can lure one monster from a pack, I can (could, I'm in NM now) melee him dead and it's easy from there, except for Diablo.. I can't beat his ring of fire.. Decrep slows down the domination but its still inevitable as there arent evought corpses around him to keep my army up.. but then again I was able to beat him with the help of one other similarly leveld Barb (I've been playing solo.. for the most part. I had to get help with duriel and Diablo but thats it (I didn't have decrep on duriel so he made short work of my weak skellies at the time)