What defensive spells would you want to cast, if you were in Dumble's army?

[gbxt7]

What creatively witty (serious or humorous) defensive spell/s
would you most want to cast against your opponents (in either
the Harry Potter world, or in our real one), if you were one
of the Dumbledore's army's most senior members, or were its
most senior member and please say why, for any spell/s named.
It can be any witty spell/s you wish was used in the 7 HP books,
but which for some reason never was, or maybe it was, but it was
used mediocrely in your view, in the 7 books.

gregbaxter7