without dying from boredom?

They bot.

You HAVE to have a group or channel you constantly level with. If you have a group time flies. Leveling alone or joining random pub games would be impossible, slow, and very boring. which is why ppl end up b0tting

1. Classic Rush if possible / Glitch Rush for Assassins/Druids

2. UT for Lvls 1-93

3. 93-97 Baal Runs + killing Nihla before 1st Wave + possibly Dia (ONLY of you don't miss waves... coz' waves' EXP > Dia 'til 98)

4. 98-99 Killing Nihla alone, Dia in 3Player-party + Baal in 2Player- or 4PLayer-party...

Ofc you need a good party/friends to do this effectively and without getting bored to death

they bot

if you got a private team going, having someone looking for exp shrine for you in every game also helps

You need to get to 98 first then you can lvl to 99

this is how i did it

Follow some runs for ages and do Diablo then Nihlathak then (if the group is slow) you might even get to do minions and then baal.
Gfg exp in full game
Just do about 100 runs per day for 45 days to get 98-99...
Basically, you have to spend all day everyday of the entire summer holidays to get from 98 to 99...

The two lvl 99 chars i had 1 was pre .09 patch,...was ez to get to lvl 99 cowing etc

The other i payed mass fg for lvl service

nn PvM lol