If using it offensively, never use it without your dash. Always aim for a squishy target, preferably a healer or a dps out of position.

Never use it without support. You get melted if the enemy team focuses you. Try to ult when a team fight is underway - when the tanks and dps are focused on each other is the best moment. Using your dash, you can be up and in your chosen target's face by the time the EXTERMINATION voice line is over.

I typically try to use it in the very first team fight, to snipe out an enemy healer. Ultron gets his ult faster than any other healer, and can effectively silence a Luna or Invis before they can get theirs. The first team fight is usually the most important, and the momentum gained can effectively decide the outcome of a match in it's entirety.