Question: Guardian boons can't be used to hurt others, and you've talked about Unseen Shield before, but what about something like I Say Thee Nay? Is a Scion forbidden from fighting back against someone trying to hurt their charge? That seems awfully restrictive.
Of course not. You can fight back against an enemy all you want. You just lose the benefit of I Say Thee Nay! as soon as you do, because you've decided to take an action that involves attacking someone else instead of devoting yourself to defense.
The boon is restrictive, and it's restrictive very much on purpose. Adding your successes to your DV is bananas-cherry-pie craziness level of powerful; as we recently noted in posts about maximum defense capabilities, a god with this boon who uses Animal Aspect easily adds over 100 to their DV, making them literally untouchable to anyone who isn't completely frontloaded for attack (not to mention a lot less hurtable, thanks to cutting over 100 off the threshold successes that would otherwise have become damage dice). And not only do you get that DV, but you also get to give that DV to someone else, something that no other power in the game can duplicate and which is incredibly useful for protection of vulnerable people or other party members. I Say Thee Nay! is one of the few ways in the game, along with the Wits knacks that allow Scions to force enemies to target the user instead of others, that Scions who are not yet gods can reliably protect friends, loved ones and other important people from being hurt in the constant high-stakes craziness of the Titan war.
More to the point than power level, however - that can always be adjusted, after all - is the point of the boon, and the Guardian purview as a whole: it's for protecting other people, period. It's not for making you better at fighting; it's not for making yourself invincible; it's not even for using to get rid of enemies who might hurt those you want to save. It's a purview that is conceptually built around the idea of defense and defense only, and its powers should never be used to enable damage or danger against anyone, friend or foe. Guardian boons can never be used to hurt others, period, and that means that the second you start doing that, whatever Guardian boon you already had active stops functioning.
Just like you can't take a shot at anyone and still stay safe inside an Unseen Shield, you can't use I Say Thee Nay! to protect yourself while wreaking havoc on the battlefield. If that were the case, it would be a mandatory boon for all combat characters thanks to its incredible power; everyone would buy it specifically to hurt other people without being hittable in combat, which is the exact opposite of what the Guardian purview is supposed to do.
So yes, if you want the badass DV bonuses of I Say Thee Nay! for yourself and your target, you can't attack anyone, even if they're trying to attack you. Guardian is here to protect you and your friends, not to make it easier for you to kill your enemies, even if said enemies are the ones you need protecting from. The boon enables you to be incredible at defense only, and is not intended for any other use.
It's true, it would be incredibly awesome to mow down enemies on the battlefield while being almost completely invincible thanks to your crazily boosted DV and ensure that Unconscious Comrade A was similarly teflon, but it would also be broken as fuck. So you can't do that. Sorry.
There are a few minor exceptions, but as a general rule, any time you think you can use a Guardian boon to hurt, kill or make yourself better able to hurt or kill someone else, you're probably doing it wrong.