Finding both yourself and your opponent in Guardia di Testa, a natural attack is a mandritto against the face, flank, or leg. Manciolino writes,

throw a mandritto to his face, or flanks, or if you wish, to his leg.

This is not what I would call a high percentage shot. In fact, it is downright risky against someone who is resting in Guardia di Testa. But if you catch someone just entering that guard you should be able to pull it off.

Counter

The counter for this demonstrates the principle of contraction and expansion.

against the mandritto to the flank, leg, or face, you can withdraw your right foot behind your left into large pace, and in this tempo you will avoid the mandritto however it may be done. And finding yourself in coda lunga alta, thereafter you will extend a thrust to his face, and in this extension you will step forward with your right foot into large pace, giving him in this tempo a mandritto to the face.

To begin you void the attack by stepping back. Make sure you go all the way back so that there is no doubt that the attack will miss you. This is the contraction.

The expansion occurs the moment his point leaves your presence. Throw an earnest thrust, but don’t expect it to land. We found its true purpose is to clear an opening and unbalance your opponent so that the mandritto that follows will land.

It may be tempting to throw the mandritto to the leg, but I would caution against it. This may leave your head exposed.

If he counters the mandritto to the head by backing up and parrying, press the attack. By rushing forward you can further unbalance him. Eventually your blows will land or you will close to grappling distance and easily knock him down.

If he manages to parry without being forced back then break measure or proceed with caution. It is rarely wise to rush a stable opponent.