there is one way to kill it without bugging as I know and I have succeed kill belial. If you have enough patience to stay on the computer at least 5 hours, you can do it. if your equip + 8 or above it's less time consuming.
Preparation : 1 barbarian (90+ is better) 10-14k hp after beast transform , 1 archer, 2 clerics ( 85-90+ ) if you have 3 clerics that's perfect :D, and two random class ( I suggest archers or venomancers). exp=0%, bring about 2-3 hp+mp charm for backup,
Method: barbarian hold the boss, 2 clerics heal him at maximum range, and make sure that the distance between 2 clerics is equal to the range of the "ressu..." skill. the others stand at 2 side of clerics and one in the middle try to stand 15meter away from clerics. when the boss's hp goes down to 30% he will do random spell attacks. 1 hit 1 kill>:) . clerics have to res as soon as possible.
p/s: this topic is thousand years ago why people dig it up :-//