I do this alot. Not doing it with a knife but using the dispenser as a shield and a weapon to kill the engi. I have fun doing this in dustbowl map 2 cap point 2 in a diff way. When they are beside the building that you usually drop from in that little corner. I'm sure you know where im talking about so that the SG covers the point. You can get by the point and kneel below the steps on the far side and undisquise to make the sg shoot at you and not hit you. Its good when the engi is at the first point and doesnt know whats going on. When it runs out of bullets get ready to cap the point.