Due to Balrog in general being outdated and seem to have no plans to be updated anytime soon the content isn't really worth doing but for any newbies who want to at least try it I feel it should be documented nevertheless. I personally had to look up a guide when I first started so hopefully this will be helpful to someone. Nexon have a bad habit of changing Balrog's related content without any notice so level cap, level requirements, number of entries and such may change seemingly at random without anyone knowing. So if this guide seem wrong at any point in terms of those related factors then please keep that in mind. Overall the content itself should remain the same until Nexon finally decide to revamp it though.
Balrog is found deep in Sleepywood's Dungeon. You go through all of the Drakes until you reach a temple area call "Another Door" which have a few NPCs and a store sitting there. You take the path down until you reach "Bottom of the Temple" where the main outside area of the boss is located. It is worth noting that you can't teleport out of that area or the stair area, if you ever wish to leave by teleport you will have to make your way to the "Another Door" map first.
Once in the correct place you will see a NPC named Mu Young who will be the NPC that allow you into the boss room and if for whatever reason you need to leave allow you to do so from within the boss room itself. Regardless if you disconnect, leave or die to the boss (and get teleported back to town) it will count as an entry so keep that in mind though.
The boss himself require a party (even if just by yourself) but even at the lowest possible level can be done easily solo though depending on class tankiness/regen will decide whichever you may need to use potions or be able to kill the boss virtually without any harm. While there isn't even a recommended range for this extremely easy boss the hard set requirement is Level 65+ and as of writing this have no level cap. While you won't find players willing to join a party for such an easy unrewarding boss you can have up to six members in your party at once which includes yourself. You are allowed 7 entries per day.
Balrog does have a prequest somewhat though none of it is required. They are more of just quests that so happen to relate to Balrog himself.
Manji can be found in Sleepywood and simply want you to check up on Mu Young.
Within Sleepywood's cave where the normal Drakes are you can find The Mysterious Note That will ultimately cause you to fight an extremely weak version of Balrog once you ask Mu Young about it. This will set up an annoying unrewarding GMS exclusive quest of killing Balrog 200 times. Yes you read that right, that wasn't a typo. Tristan's Spirit will tell you of your progress in the portal left of the bossing area near the portal where you exit Balrog's reward room from.
The final reward beside a month of waiting to kill Balrog is an extremely low tier medal. STR +2, DEX +2, INT +2, LUK +2 are the stats and as you can see literally any theme dungeon related medal will outclass it.
Suspicious Man have a quest to give you an objectively useless scroll (Balrog's Accuracy Scroll 30%) after collecting 20 Balrog Leather for him unless on a Reboot server in which case he will reward you with 300k mesos instead. Otherwise he will use the leather to craft special scrolls of your choice for Balrog's Leather Shoes or Balrog's Fur Shoes. For Reboot players leather have no real value after completing his quest.
Before I get into how to fight the boss I feel knowing if it is even worth your time is more importation to know first. Rarely does anyone do Balrog and for good reason. Assuming you had killed the boss you will be put into a special room where you may use your basic attack to break open a red gemstone in the middle of the room. This gemstone will drop Balrog Leather (you will also get guarantee Balrog Leather once you leave the room) along with some healing items, possible soul shard (extremely low chance), Balrog's Leather Shoes, Balrog's Fur Shoes and lv80-90 equipment.
Balrog's Leather Shoes have STR +2, DEX +2, INT +2, LUK +2 though stats can be higher or lower depending on luck. Balrog's Fur Shoes have the same stats but can level up twice adding a possible point increase each time per stat. Overall something you will quickly replace at some point. The soul shards aren't worth collecting as the souls just give you a powerful attack with a 120 second cooldown between use. There are better and much easier to obtain souls in the game.
I honestly can't recommend this boss as the rewards aren't worth the time spent so only do the boss if you are a completionist.
How To Fight Balrog
As soon you enter the room you will notice many things.
1. The boss can attack you from anywhere and will prevent you from dropping items from your inventory and such. Bringing out your familiar in advance is highly recommended.
2. The boss HP won't decrease.
3. You can attack both the hand and the face.
You will need to do a small amount of damage within a 3 minute time limit or else be kicked out of the room and wasting that entry. 50k total damage is all you need though and in some cases players might be able to do that in a single attack. As for the rest of the time you just have to wait and keep yourself healed if necessary.
Once the timer hits 16:59 and assuming you had dealt enough damage to stay in the room Balrog will enter phase 2 and free his other hand. I generally buff up around 17:10 depending on how many buffs the class have in advance. Balrog does have a skill that remove all of your buffs so you may want to pay close attention to that, he may never use the debuff or spam it multiple times within a minute. It seem to be based purely on RNG. Attacking the head or hands don't make much of a differences though the more body parts you can attack at once the more overall damage you can deal. If you can only attack one area then go for the head as sooner or later Balrog's hands will get sealed and can't be harmed. At this point Balrog health will start going down as you attack him.
For the most part Balrog is just a damage sponge of a boss though there is a point where he will start casting spells. Once Balrog get between 1/3rd and 1/4th health (when the health bar go slightly pass the timer) he will cast Damage Reflect on himself which will last around 2 seconds, long enough for anyone who didn't noticed to die. Damage Reflect cause you to deal damage back on to yourself while the monster only lose 1 health per hit. Extremely dangerous on that note. The animation have his horns glow red with a fire-like animation and have a magic circle in front of him as seen in the above gif. Most of his animations are like that but once you know the exact animation you should have no issues pointing it out. Note only Balrog's health matters for this, not the timer though once you learn your average time for that character you might be able to use it for future fights. As long you stop as you notice the animation then even the weakest defensive classes can survive. After that point he may cast another spell which is Zombie. Zombie cause potions to only heal half the normal amount and make Priest's heal skill deal damage instead of healing potentially killing the player the Priest meant to heal. He will go between these two spells so assuming Zombie is nothing for you to worry about then just keep attacking after Damage Reflect fade away. Unless your class is extremely weak Balrog should be dead long before he get the chance to cast Damage Reflect again.
Once you learn his mechanics he is extremely easy boss to deal with. Mostly a waiting game than anything. Honestly might even be worth doing if it wasn't for the three minute wait time per run. 600 minutes of just waiting over a course of a month just for the medal if anyone was wondering.
Balrog Bossing Guide
Guide in 'Bossing Guides' published by Zelly, Feb 6, 2018.
How to do the Balrog related content and the possible rewards that you may obtain from doing such.
Changelog1.1 - Fix a few mistakes since creation, add a few finer details and gave the exact related quest reward for reboot players thanks to Zerovii.