How do you complete the Team Fortress 2 "Line Them Up" challenge?

I know this challenge must be solvable, but it seems impossible to me at this moment, since no matter what I do, it takes 3 actions to defeat the first three wizards (with the Heavy, or with the Medic boosting the Demoman) and 3 actions to defeat the last three wizards (by moving the Heavy over and attacking twice).

How do you complete this challenge?

Solution 1:

Use the heavy and shoot the first mage, then the second, then the third mage. Select the demoman and step on the second mages body. Choose the demoman again and walk one spot further. Lob a grenade at the last Mage, then the Mage in front of him

Solution 2:

None of the other answers are correct, they all leave one Wizard left alive with 20 HP, thus failing the challenge. Here is the correct solution:

1. Attack the 2nd Wizard from the left with your Heavy.
2. Attack the 3rd Wizard from the left with your Heavy. (3rd Wizard dies)
3. Attack the 1st Wizard from the left with your Demoman. (1st/2nd Wizard dies)
4. Move your Demoman up two spaces.
5. Move your Demoman right one space.
6. Attack the 6th Wizard from the left with your Demoman.
7. Attack the 5th Wizard from the left with your Demoman. (4th/5th/6th Wizard dies)

Voila. You have completed the challenge. Cheers!

Solution 3:

Kill the 3 first using the machinegun guy then use the bomber on last 3. You dont need to use the item. Trick is you point at different mages using the gunner. Hope this helps!