How do you defeat the roadblock giganto?

The simple answer is that you can't. He is immortal until the timer runs out.

The Giganto Barrage is just there to score points, you have to fight it 5x to get the rewards, no matter how much damage done, it will remain until the end of the timer

Combine your workers until you start getting bigger higher ranked ones and you will have enough resources to beat him. You can only combine workers that are the same type... jackhammer vs pick axe