Pierce vs guardian a unit with immune to pierce

[kiryuCPR]

if i attack a unit, with pierce keyword in my pool that is immune to pierce, but another unit without immune to pierce uses the keyword guardian, do i (the attacker) choose where my pierce keyword would apply? and what is the timing of this 

[Seth]

Hello,

As explained under the Pierce X & Guardian X keywords in the Core Rulebook, the attacking player may decide to apply Pierce to the defense dice of the Guardian unit. This will be resolved prior to rolling dice for the defending unit.

While a unit with Immune: Pierce is defending, Pierce cannot be used during the Modify Defense Dice step; the defending unit having Immune: Pierce will not grant that immunity to the unit using Guardian during the Modify Attack Dice step. If the unit using Guardian itself has Immune: Pierce, its block results cannot be canceled while rolling defense dice as a result of the Guardian keyword, as detailed in the Immune: Pierce entry.

Hope this helps,
Seth