Sudoku is a game of logic  the object is to complete the grid such that every row, every column, and every 3x3 block contains the digits from 1 to 9. No mathematics is required to solve the puzzles, just pure logic and reasoning.
When your cursor is on the Sudoku grid, it can be navigated in the following way:
CTRL + arrows
Move around the grid (you might need to press SHIFT as well)
a
Autopencil marks
p
Show a pinned square
SHIFT + #
Show squares with a <#>
There are six levels of difficulty, and various techniques are potentially required to solve Sudoku at each level, however, any particular Sudoku doesn't necessarily require all of the potential techniques for that level. The difficulty level is also very tricky to gauge as one person may find certain techniques easier to use than others. The hardest of one level will probably be harder than the easiest of the next level up, so there is a certain amount of crossover between the levels. Each BrainBashers Sudoku has a list of potential techniques that might be required to solve that Sudoku, some of which you might not use, as you may spot something which changes the solution path and you use other techniques.
Note: this list isn't exhaustive, these techniques are just my suggestions to help you solve BrainBashers Sudoku.
Difficulty Levels:1 = Very Easy, 2 = Easy, 3 = Medium, 4 = Hard, 5 = Very Hard, 6 = Super Hard:
Techniques 
1 
2 
3 
4 
5 
6 

Y 
Y 
Y 
Y 
Y 
Y 

Y 
Y 
Y 
Y 
Y 
Y 

Y 
Y 
Y 
Y 
Y 
Y 

Y 
Y 
Y 
Y 
Y 
Y 

 
 
 
Y 
Y 
Y 

 
 
 
Y 
Y 
Y 

 
 
 
 
Y 
Y 

 
 
 
 
Y 
Y 

 
 
 
 
Y 
Y 

Unique Rectangles 
 
 
 
 
Y 
Y 
 
 
 
 
Y 
Y 

Unique Rectangles 
 
 
 
 
 
Y 
 
 
 
 
 
Y 

 
 
 
 
 
Y 

 
 
 
 
 
Y 
Assistant's Features
Backspace
The key 'Backspace' has a reserved action, and in many browsers it acts as a 'Back' button. This may mean that your browser will go back a page, and leave the Sudoku Assistant, if you are not careful with its use. I suggest that you only use the 'Delete' key to remove digits.
Pencil marks
What is a pencil mark? Pencil marks can help you keep track of your own thinking, simply enter more than one digit / letter into each square, just like you might do if you were using pen and paper. The size of the digits / letters is smaller when there is more than one digit / letter in a square. A square might contain more pencil mark digits / letters than can be shown.
'Autopencil marks'
The system can fill in the pencil marks for you automatically. Each square is checked to see what digits can possibly go there, if only one possibility remains, then that digit is filled in. You can manually change the pencil marks and run the option as many times as you like. For difficulties above Medium it also autoruns until no board changes take place. When using the blank or loaded assistant for nonBrainBashers Sudoku, remember that autopencil marks might complete the Sudoku for you and spoil the fun, however it doesn't autorun, so you may have to keep running it as the system changes the board.
'Show errors'
If this option is selected, correct digits are shown in green and incorrect ones in red. This is useful for checking your progress midsolution. This is only available for BrainBashers Sudoku. The system only looks at squares with a single digit, and does not consider the validity of the pencil marks.
'Show duplicates'
If this option is selected, duplicates in a row, column or 3x3 block are shown in yellow.
'Show squares with a ...'
All pencil marked squares that contain the selected digit will be highlighted. This is very useful when looking for a digit which is missing from a row, column or 3x3 block.
'Show a pinned square'
If one of the rows, columns or 3x3 blocks contains exactly one instance of a digit, all pencil marked squares that contain this digit will be highlighted in green, with the pinned square highlighted in yellow. This is very useful for finding pinned squares. If no row, column or 3x3 block contains a pinned square, any highlights will be removed. This option requires some pencil marks on the grid.
'Show Potential Techniques'
This option will show you which techniques might be required to solve the current BrainBashers Sudoku.
Print
Only BrainBashers Sudoku can be printed (the starting grid, without pencil marks).
Undo
Undo takes the Sudoku grid back to just before an autopencil marks was run.
Note: BrainBashers has a Dark Mode setting.