World 8

Here's a nice walkthrough of Final Castle where Bowser lives:

If you run straight right, you'll time it so that you'll avoid all the
Fireballs so you can enter the door at the end straight away. The right brick
Has a power-up and the central one a P-Switch that will turn the room upside
Down. Either the door on the right. In this area, go right past the three
Burners and take out the Dry Bones. Drop down and past another two burners,
And across the brick when the burner is inactive. Jump across by the edge of
The next burner to avoid the last one, and continue past the others to the
Door on the right. Run right and trick the giant Thwomp into falling, then
Run back and run and jump underneath him as he rises. Hit the third brick
From the left for the P-Switch, then right and up (? Block has a power-up and
Activate to rotate the room. Trick the Thwomp again and enter the door left.

Head all the way to the far left past various burners until you reach a wide
Spike pit. Run and jump across it and hit the brick for a ? Switch, which will
Make a moving platform appear in the area next door. Now head all the way
Back left to where the Dry Bones was, but instead head up the ledge and
Run and duck under the small burner by the door and enter. Ahead are there
Thwomps, their positions indicated by the gold blocks underneath. Like with the
Giant one trick them and then progress past. After the third, make it drop then
Jump left and to the end for STAR COIN 1. Go back now, past that third
Thwomp and drop down on to the bricks below. Head left and get the P-Switch
From the second row of bricks and rotate the room. Go up, right and past the
Other Thwomp as it rises, over the gap, past the Dry Bones and into the door.
If you activated the moving platform earlier, it'll be on the left so you
Can get to the door, where STAR COIN 2 is along the bottom passage. Enter
The door above to reach the next area (same as the normal route by the way).
(CHECKPOINT).

Head over the gap and get a power-up from the ? Block; here you have to go in
The right route to make the next route appear. Firstly run straight across the
Bottom under the ledges. Jump over the various gaps and the second is across
The top of the ledges (run across under the spike pit) then go through the
Middle of the ledges to progress. Over the gaps and you'll see STAR COIN 3
Behind some bricks, then the Last Boss Door follows!

FINAL BOSS: BOWSER AND BOWSER JR.
You'll face both Bowser and his son at the same time! To begin with run left
And concentrate just on taking out Bowser Jr. The same old thing applies here;
Avoid his runs and ducks into the shell, get the shell he throws, kick it at
Him then Ground Pound him (twice in total) to get rid of him. You'll just have
To avoid the blue homing fireballs as you go, but they move slow enough anyway.
Once he is done it's Bowsers turn. His attacks include red fireballs spread
Out, which are easy enough to dodge and multiple blue homing fireballs.
Stand about two Mario's lengths away; if he spits out any fireballs it'll
Go straight over your head, plus it gives you an easier chance to run past
Him as he jumps high. Just hit the red switch at the of the bridge to finish
Him off!