Here is an example of working a very hard puzzle.

Try filling in numbers 1 through 8

Now fill in the 9s and then more 3s.

Now fill in 4s in RM Box and then 2 in the last open
square and in the middle box.

next fill in 7 in the lower middle box and in the upper
middle box.

Now if you notice the upper middle box with the 3 7 8
and 9 put into the upper part of the squares. They have these 4 squares
“locked up” and no other numbers can go in them. If you see you can move
from square to square following the numbers in a circle going from 3 to 8 t
9 to 7 and back to 3. If one of those number is ever located, the other
three will fill the rest of those squares. So that means the remaining
three squares must contain 2 4 and 6. 6 and 4 lock up two of the three
squares so 2 goes into the third square.

Fill in the 2s and 6s

Next the 6 and 9 in the right middle box, the 9s in the
right upper box and then the 3s have to go in the two remaining squares with
the 8s. The threes show you which square the 3 goes into in the middle
upper box and the 7, 8, and 9 follow.

The rest of the squares you will find easily. Fill in
squares in the middle middle box. Then fill in the Left Middle box and the
lower right box.

Then fill in the rest of the puzzle.