to do it you need to have 2 accounts (one with lvl 60 char and one with char you want to powerlvl)
-get on 60 lvl char and go to belial (normal / nigtmare or hell, depends on your lowbie char lvl of course)
-fight belial, and before you kill him join with your lvling char (you must join before you kill him, you can do that even at the start of the game but he will be harder, up to you)
-kill belial with your lvl 60 char when lvling char is in the party, after the boss is dead you need to port to your 60 lvl char (use banner in the city - this is not needed if both chars were in the game from the begging, only if you joined the game during belial fight)
getting xp part:
1 turnin
-leave the game with your lowbie
-hit resume game (do not join - RESUME) on your lowbie and click on soul stone, then go to the town and give q back to tyrael - now you got xp (yay!)
2 turnin
-join back to your 60 char game with your lowbie and leave with your lvl 60 char
-click on soulstone and turn quest to tyrael with lowbie char
-leave the game
3 turnin
-resume game with lvl 60 char
-join this game with your lowbie and click on the soulstone, then leave with your lowbie char
-resume the game with your lowbie and turn the quest to tyrael
-leave the game
4 turnin
-join your 60 lvl char with your lowbie again
-leave the game with your lvl 60 char
-turn quest with your lowbie
-leave the game
5 turnin
-resume game with your lvl 60 char
-join this game with your lowbie and turn quest back in last time
repeat form the begging
credits for 4 & 5 turnin to 20kilo, Wyvernrider and shadowsx for help with testing
3 accounts setup
You can do it with 3 or 4 chars if you want to get much more XP per run
Example 1 of 3 acc setup:
1 turnin
make game with 60 lvl char -> join with lowbies -> kill belial -> leave with both lowbies -> on lowbie no1 make single player game -> turn the quest -> leave the game -> make game with lowbie no2 -> join his game with lowbie no1 and turn the quest (this way you're getting xp on lowbie no1 again) then just go to the step 2 and repeat.
Example 2 of 3 acc setup (for advanced) :
1 turnin
make game with 60 lvl char -> join with lowbies -> kill belial -> leave with both lowbies -> on lowbie no1 make single player game -> turn the quest -> leave the game -> make game with lowbie no2 -> join his game with lowbie no1 -> pickup soulstone -> leave with lowbie no1 -> turn quest to tyrael on lowbie no2 -> make game with lowbie no1 and join it with lowbie no2 -> turn q to tyrael
go to the step 2
I won't be posting 4 acc setup as if you're smart enough to get it working on 3, you can also get it working on 4
Tips:
if you join during the fight you won't be in the chamber, but you just need to port to the chamber (using the banner) after fight its done, and then leave the game
Tyrael turn in is real xp, not the soulstone. you're getting xp x3 for tyrael turnin (you loot the soulstone only twice)