Before you start your run, press the "select" button on your pad and give yourself 5 lives and 3 bombs, while setting the game on easy mode. This will not void the achievements and is valid for both Vasara 1 and 2.
At stages 6 and 12, you'll want to turn on a second controller and log in using a spare account to make use of the glitch which allows you to keep continuing provided at least one player remains alive at any one time.
Make sure the 2nd player dies faster than you and use those extra bombs to cheese the bosses. Whenever my dummy died, I launched a bomb to give me enough time to take my eyes off the screen and press continue on the 2nd pad. Worst case scenario, you'll simply restart at the beginning of either stages 6 or 12, depending on where you hit the game over screen.
The last 6 stages are actually the same as the 6 before, only way harder. After beating Nobunaga's true form at the end of stage 12, both players will unlock "The Long Road".