Slime Eel is the boss of the fifth dungeon of Zelda: Link's Awakening, the Catfish's Maw.
There's a useful tip to make this boss pretty straightforward - but that's not to say you won't be open to damage if you're not careful.
If you're looking for more assistance, our Zelda: Link's Awakening walkthrough can help.
Slime Eel boss strategy
The Slime Eel has a pretty simple strategy to follow, as long as you are patient.
Stand in the middle on either the left or right wall, allowing you to dodge all the tail attacks without moving.
While there, you'll notice the boss's head will emerge out of one of four holes in the wall. When it emerges from the side you are on, as it's mouth is open, use a hook shot on it.
If successful, it will drag the body out, revealing an exposed glowing weak point you can swipe with your sword. You can hit it multiple times before it withdraws and repeats the process.
Provided you are patient, and avoid the swinging tail as you step out to use the hookshot, you can get through this boss with little difficulty.
The only other thing to be aware of is when you hookshot, you'll occasionally receive a much smaller version of the boss, which thrashes out until it explodes. When it does this, quickly equip and hold your shield to protect yourself.
When you've inflicted enough damage, the boss battle will end and a customary Heart Container is your reward.
Head to the next room to receive the fifth instrument - the Wind Marimba.
Our Zelda: Link's Awakening walkthrough and guide can help you with the critical path, including the Eagle's Tower and Turtle Rock dungeons. There's plenty to be done on the side, too - including Zelda Link's Awakening Heart Pieces, Zelda Secret Seashells and the Zelda Link's Awakening trading sequence. Finally, there's also the new Zelda Chamber Dungeons feature to unlock and complete.