Mittelborg: City of Mages (UP)

Very easy after the tower has taken damage you can sacrifice a mage to repair it.

You can do this after the tutorial and then die and revive after that or later after some lifes you will have enough mages anyway to make some sacrifices.

Be aware that the whole game is buggy and seems to treat your first 2 Lifes like a tutorial without the ability to unlock achievements.
If you fulfill this condition during the tutorial you need to delete your save, redo the tutorial and do the condition again