If I'm in nightmare already, what is the fastest way to get to level 40?

Level progression is not good in this game. Something is really broken with levels 20-24, but there are a million guides online about it.

The fastest method from level 20-24 is normal cows with someone carrying you.
The second fastest method from level 20-24 is solo tombs in a full game if your build allows it.

Get an enchant sorc to boost you with 6k fire damage

what lvl are you ?

normal baal is fine to lvl40

If you wanna do it by yourself best char is nova sorc. Get her basic gear like peasant crown, spirit sword and viper. The key is to farm alone in 5+ players games.
1-15 farm rakanishu and tristram.
15-24 flayer jungle act3. Theres a lot of flayers and nova can kill them very fast, exp is gained extremely fast.
After level 30 you do solo baalruns in player 8 games. With few points in static field you can easily kill all baal minions and 1 baalrun is 2 levels.

1-40 is easily possible in one evening

i thought you can't have completed norm baal q if you wanna be bumper

Fastest way immo --> rush lvl 1 char in classic to start a3 hell, then convert to expansion.
Continue progress to A4 Hell and leech on Hell CS runs until lvl 21.
Now do some cowruns in normal from 21-25
After 25 continue with Hell CS runs until lvl 40+

bumper is ready