-norm baals, no shame in anything up to 55 in my opinion...possible to go higher without much effort too, but if you can't handle A1-3 NM by 55 with even garbage gear you might need to rethink your build ^^
I'm talking about solo in 8 player games of course, not public baals
-just play through NM solo...play through doing all quests until you get to NM baals, you should pick up a decent amount of levels on the way.
-find NM cow games (GL) and leech
-leech (and/or party) to NM baals and then leech in NM baals...this is the fastest, but is also the most likely to end up with you tppked
I usually rarely ever nm baal till high 60's low 70's. Not sure if your strong enough, but if you can at 40 hit Act3. Very good exp with flayers, at around 50 i then start going to Halls of pain, VERY good exp in their, especially if the right mobs spawn.
out of all the characters ive quested, lightsorks are probably the hardest (of necs, fire/cold sorks, trappers, and barbs). lightning really drains ur mana, and it doesnt particularly deal insane damage either.
lvl 40 soloing? i suggest doing more norm baals. i did that until 45 so i can wear my +3 sork ammy. go for 50 if you really are untwinked.
slap on an insight on any polearm before u head to nightmare. really helps when teleing/killing.
what i usually do (and this is true for most of my questers) is to get to a4 asap, then lvl in pubbies, doing chaos solo. again, this is harder for light sorks, but is very doable. find some games that have ~3 peeps in it. 8p for light sorks is a bit too hard.