To get to the the first boss battle, you must collect the required amount of clover pages.
During the fight with Master Mittens, he will summon 13 enemies and fire multiple cannons. (I recommend jumping over the fireballs.)
At the start of the fight, he'll summon 7 enemies. (These can easily be beaten with 1 tailswipe.)
Grab the coins fast to get a diamond.
32 cannons will rise. (16 on the left and 16 on the right.)
The left cannons will always fire twice except for the final attack.
Grab the coins fast to get 2 hearts.
1 enemy will be summoned. (Hit it while facing Master Mittens, this will cause it's head to hit Master Mittens.)
19 cannon will rise below Master Mittens. (They shoot bouncy fireballs.)
2 enemies will be summoned.
Cannons will return.
3 enemies will be summoned.
You have now defeated Master Mittens and unlocked Veggie Village!