Engineer rush

From TF2 Wiki

The Engineer rush is normally ineffective except in specific circumstances. If the Engineers all equip a sentry ready to lay down and can all start them building at the same time while switching to the shotgun this can be effective. Engineers lack high direct damage attacks and sentries take time to build, so mobility is hampered and an enemy ubercharge can cripple your rush at setup time. If a window of opportunity arises however, a line of sentries can be set up to take out incoming enemies allowing you to push forward if the enemy team are scarce, though ultimately this is still less effective than a well-balanced team.