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".
With 2 union attack you don't need Cu Chulainn or riryoku.
Blue Eyes + Dino fusion = 5500 attack
So double union attack those monsters' attack onto Vassal and you're over 10000.
Replace them with more dinos (Uraby) for a more consistent starting hand, or more spell/trap removal I guess.
You do know Mako uses lost blue breaker as spell trap removal? To claim he doesn't in the section about wonder ballons is misleading