Fire king

If you attack with your last monster then activate onslaught of the fire kings can you still attack?

You can only activate it if your opponent have monsters while you don't. For example if you attacked with a monster, and the opponent responded with a card like Denominational prison for example, and banished your monster, which caused you to have a field empty of monsters, then you can activate Onslaught of the Fire Kings.

You have to have no monster on the field to activate it. So in your situation, it depends on if your opponent has backrow cards like Mirror Force, D. Prison, Evacuation,...