Not a hard one, though it is annoying that there isn't a save spot right before it, you'll have to backtrack a little back up after opening the doors to save.
-Note 0: You should have maxed out your Shampoo and Silky Cream by now. Also, have Bubble Shield and Super Monster Milk. Super Pike Ball may help too.
-Note 1: It attacks by ramming (Cannon Jump to avoid) and has projectiles (Use Bubble Shield, it DOES work against these ones)
-Note 2: The button pattern is random each time.
-Note 3: The Steel Maggot has 400 HP.
-Step 1: Use your Cannon Jump to get above it as it rams at you.
-Step 2: As you are jumping over it, make sure to hit one of the three buttons on top of it.
If it is the correct Red Button, it will turn Green and you can then try to get the next one.
If it is the wrong Red Button, it will reset the buttons and fire off a spread of weak shots. (You can't hit a Green Button, so only the first two are a problem)
Repeat Step 1 and 2 until you get all three buttons green, at which point its tail (the only weak spot) will be exposed and it will slow down a lot for you to attack.
-Step 3: After you have hit it a few times or several seconds pass the tail and the buttons will reset, so you have to repeat the previous steps.
-Step 4: After a couple rounds it should be defeated!
Because of the random button pattern+reset, this fight can take a bit longer, especially because it's not the easiest thing to aim the cannon shot.