With your build order your best bet is to make pg first and wait for him to spread his units and stage a sneak attack with your pg and engineers. Together they can kill the unit fast enough and you can steel it with engineers. Keep in mind this only works if he is busy capping your points or engaged in something elsewhere. It also depends where he has it positioned.
My problem is always that I'm sending my units as soon as they are reinforced. Sometimes I have to take time and plan an attack like this one. Quite difficult I think but when successful this hurts the opponent like hell.