Archons invulnerable saves

So previously you could choose to use the normal 5+ save instead of the 2+ to preserve the shadowfield but is that also an option now that he has 2 invu saves? Can he use the 6++ from PfP instead of the shadowfield..?

You are free to choose between his 4+, 6++/5++, or 2++. Core rulebook, page 222.

Da Rulz wrote:

Some models have an invulnerable save. Each time an attack is allocated to a model with an invulnerable save, you can choose to use either its normal Save (Sv) characteristic or its invulnerable save, but not both. If a model has more than one invulnerable save, it can only use one of them — choose which it will use. If you use a model's invulnerable save, it is never modified by a weapon’s Armour Penetration value.

You have to specifically fail that 2++ to pop the bubble. Ive used bolter fire on my 4+ armor save just fine.