A lot of people seem to get stuck on this particular level. The wall is so huge you'd think the progression curve is stupid. You face 4 levels of mobs that depletes your entire HP bar in an instant and if you get through that you'll face Demon Lord Baal that if your stats is not optimized for the run he would just erase you even at 100% HP.

it is not a wall that you need to grind (or maybe a bit) but more on how you allocate your stats that will make you beat him. You'll also get "Boss Life" achievement with this guide after beating him.

Defeat Demon Lord Baal

I beat Baal with this stat allocation. Now others have certainly did the run with fewer stats than mine and some still figuring this out what matters is the stats allocation.

As you noticed I put a little bit of Crit and Atk Spd then spammed Atk and Def while ignoring HP completely.

The scaling for HP is terrible in my opinion. It doesn't give much even after certain amount of points are used and you can easily reclaim the stats you lose from late game items. Better alternative for survivability is defense + the skills (Untouchable and Defensive Style) that scales it.

Atk Spd and Crit is the same way they start off with giving you 2 or 3 on each until it starts giving you 1 for Atk Spd and 0.5 for Crit. At that point, it's not worth putting more stats into it because what you get from gears will just overshadow them. The reason I allocated a bit of stats into them is because you need them to safely start your run and make the clear rate as fast and efficient as possible.

Atk and Def on the other hand is important. You can never go wrong with more damage as it helps you kill monsters sooner before they can do anything to you if combined with proper gears you'll notice it multiply like crazy. 500k to 1 million damage from just switching to a well optimized sword. Lastly, Def scales the best. They are what make you progress. There is no point in 1 hitting a boss if they can 1 hit you first. Defense fixes that problem. The more stats you put into the more damage you can endure. A lot of stats allocated into it + Defense % from gears makes the regeneration you get from Passive Heal bigger than their damage.

TLDR:

• Start with a DEF stat.
  
• Slowly allocate stats to Atk Spd or Crit while alternating it with defense. (Defense -> Atk Spd -> Defense -> Crit -> Defense -> Atk Spd -> etc...)
  
• Switch to alternating Atk and Defense for the rest of the game with all your stats. (Defense is a more priority than Atk)
  
• You don't need HP stats. Get them from gears. Find just how many you need to not get burst down then go back to spamming Atk and Def.

Furious Jump/Lightning Strike > Offensive Style > Lightning Strike/Furious Jump > Defensive Style > Untouchable/Bearzerk (if you need to scale, optional.) > Passive Heal (for maximum survivability) > anything you havent taken yet.

Skill building is pretty straight forward. You eventually max them out later in the game with enough reincarnation. I choose to max and focus on 1 first at a time because there's a huge power spike in going from a lvl 4 skill to a lvl 5. (You'll notice the boost % suddenly goes a lot higher)

• Always repeat the level you're stuck at not the previous one. You get gears on the same power as the level you're trying to finish. (That weapon I used to kill Baal came from that level too.)
  
• Rush your 3rd reincarnation so you can get your helper and level them as soon as possible. They provide neat bonuses such as "Looter" which gives you more loot and can help in finding gears with stats that you need.
  
• Always optimize your stats. Only get the Def and HP you need to survive a boss and invest the rest on Atk. Keep Atk Speed at 120+ and Crit at 40%/50% with the late-game gears they'll just increase on their own.