In the first phase, you can't deal damage to Balrog until around
19:59minutes. That's a test of Mu Young: if you deal enough damage in those
10 minutes you will be able to fight Balrog, if you don't you will get
booted out at
19:59.
The second phase starts after
19:59minutes, and you'll notice it because a message will pop up and the HP
bar will appear. From now on, you'll start dealing actual damage to the
Balrog.
The third phase starts when Balrog has around 45% left
and a second message pops up, just like the first phase. The only
difference of the third phase is that Balrog can now cast Damage
Reflection for both Weapon and Magic attacks. That means that every
time you will attack him during the Damage Reflection skill, you will
get back around 80% of the damage you dealt, and you'll most likely
die. Be careful: when the third phase starts, don't attack until the
end of the first Damage Reflection! Balrog's Damage Reflection has a
cooldown of ONE MINUTE, so you can keep attacking it after Damage
Reflection expires for around 1 more minute (probably A LITTLE less).
After 1 minute from the last Damage Reflection, the Balrog will most
likely cast another one, and so on. Again, remember to be careful here!
The
fourth phase starts when Balrog has 10% HP left. There will be no
messages to notice you of this, so be careful here as well! From then
until the end of the run, he will start casting Zombify, so all the
Clerics/Priests/Bishops MUST NOT HEAL! The effect of potions is reduced
of 50% if I don't mistake, and heal means instant death here.
All info can be found along with a very helpful video on youtube:
https://www.youtube.com/watch?v=vcSNkB5WBG8&feature=response_watch