This is not
a point of no return. You are able to pick back up before the fight after completing the game.
Voltbot has two attacks at first. He can throw out metallic diggers, or use a powerful laser. There are several generators connected to him that will show on the minimap. Use your steam punch to destroy them, three hits for each will destroy them. Each generator will drop a health/water/light pickup when destroyed. When all are down, jump on top of Voltbot's head and punch him three times.
The second phase is similar to the first, only with more generators. On the third, he adds a new attack in which he fires a continuous laser while circling the room. After this third attempt, the boss is vanquished and the game is complete.
Every gold star you get in the ending corresponds to a different achievement, while getting all four at once is another achievement.