Umi decreases the ATK of the monster to 1300 but allows it to attack your life points directly.
How to defeat Mako's deck
Destroy "Umi" field
What you should do to beat Mako is the only one thing: removing "Umi" field spell to prevent Mako's monsters from benefiting from the field. Since all Mako's monsters are expected to be used with the field, the deck does not work without "Umi".