Ranged auto attacks lets you last hit without being in Dari's range.
Furthermore, Dari is forced to last hit in melee range, soooooo you know exactly where he's going to be allowing you to land your corrosive charge. After that, open up with homing missiles. If he stays in range, murder him. If he runs, try not to push too hard, Urgot can be shut down by ganks. Buy wards, play smart, and use your laugh button as much as you can.
Start Meki and pots, rush tear of the goddess, then boots, then build Mana-moon. (However the hell you spell it.) After that, tank the hell up and be useful with your ult to your team, and "Secure" kills with your homing missiles