Wizard? He's a Warlock! :)

When Sandro is killed, I can't take the grave because they attack the imp, and spacebar doesn't work:(

Spacebar should work. Move a unit onto the grave. A window opens. The grave is in one row, the unit in another. Press spacebar so they swap places. Then drag the grave with your mouse into the unit's row.
Exit the window and the monsters won't attack the unit holding the grave. Now use another unit to lead the medic with the grave out of the encirclement :) And if that doesn't work, just take the wounded Hero, and let them kill the medic :)