Zelda 1 - Walkthrough of Dungeon 3 of Quest 1: The Manji (Zelda I NES mini)
Map of the labyrinth "The Manji": Dungeon 3 of Quest 1 from Zelda I (The Legend Of Zelda NES mini)

In this dungeon you will encounter Zols that will split into 2 Gels when hit with the wooden sword. Gels can be killed with the boomerang. You will also face the fearsome red Darknuts, who can only be attacked from behind, from the side, or with bombs (no, that's not cowardly). You need to hit them 4 times with the wooden sword. So I recommend bombs if there are too many of them. Use the scenery to your advantage too (1) — they are not afraid of the boomerang. You will encounter Bubbles for the first time. If these creatures touch you, you won't be able to use your shield for a few seconds. Since Bubbles are the spirit of death, they are "immortal".

Start by going left, take the key (1) and go up. Kill all the Zols to get a key. Go up, go left and take the compass (2). Go left again (fake out the spikes and open the door while they return to their position). Kill all the Darknuts (3) to open the bottom door and go through it.

Zelda 1 - Walkthrough of Dungeon 3 of Quest 1: The Manji (Zelda I NES mini)

Take the staircase on the right to get the raft and come back out (the Darknuts drop a bomb if you kill them all). Go up twice, take the key and go back down. Go right twice and go up. Kill the 3 Zols, take the key and go back down (there is nothing above except a key and a hint). Blast a hole in the right wall and go through it. Go up, take the map (very useful now) and go right through a hole (3). You are now facing the boss.

Getting past the Darknuts without killing any won't be easy, so nothing beats throwing a bomb into the crowd to go get the raft

The boss is called Manhandla. It is a creature with 4 claws that moves slowly and launches fireballs. To kill it, you must destroy its 4 claws or its heart (between its 4 claws). The fewer claws it has, the faster it moves. The best solution is to place a bomb and try to hit it in one shot. If you miss, there's a good chance it will still lose some claws. In that case, I advise placing bombs everywhere and praying. You can also try to finish it off with the sword, but it will take more hits (4 per claw).