Only works for ground units.

Create a long path of towers such as is normal for this type of game. Plan ahead. You need a path where if you pause the game, sell a sand bag and place another one towards the end of your route of destruction you can sell off one sand bag (creating what would have been a short-cut) and then block off your near base path... forcing the unit to double back to the short-cut.

Wait unit the enemy line is close to your base. While the game is paused and you sell the first sandbag you end up having two paths. Where the unit would have normally chosen a shortcut. Then block the road to your base with a new sandbag. The ground unit ends up doubling back.

This costs you only 20$ in the game...$40 earned for the sell off and $50 for the new sand-bag and then you do it again (sell the first sand bag and replace the other to make your path to the base longer again... and make the unit turn back towards your originally blocked path. Can be repeated and costs far less than an EMP or TNT.