That's flat
Here I'll have a fling at him that's flat. - 'And here I'll have a go at petitioning the King, that's for certain' (Bevington, fn l.21, p. 91). Hieronimo has already made up him mind: to see the king, whom he can hear/see coming, and plead for justice. As the soliloquy discloses in the following two lines, he has also decided to get even with the two murderers of his son - Balthazar and Lorenzo - at a later stage.