Something funny I just experimented with based on @Constentin Lévine's finding and the one above. I had an Ascendant/Tactician, 2 priests (SC or MC doesn't matter) and a SC Wizard, with Berath Challenge on.
1) While everybody is stealthed, have the Ascendant alone approach the enemy and quickly unstealth/stealth. Combat starts, because he's still stealthed Tactician triggers Brilliant, i.e. automatic focus generation.
2) While his focus slowly fills to the max, have him go back to the group still all stealthed.
3) As soon as he reaches Ascended thanks to Brilliant, go through a full routine of Ancestor's Memory + BDD + SoT chain-casting for the entire party.
4) The Wizard then also casts Temporal Cocoon on everyone, which beneficial effect gets prolonged by the chain casting of SoT (this is why you need 2 priests, so they can each prolong Cocoon for the other).
5) Just leave them continue to cast SoT on each other like weirdos for as long as you like (why not a few hours) until you're ready to go actually do the fight. It won't last long: you'll have tens of thousands of seconds of untargetability by the enemy, cannot-die, brilliant for everyone and ascended for the Psyblade.
6) Once the fight is over, Ancestor's Memory and BDD will go away, but not Temporal Cocoon... your entire party is now permanently untargetable by any enemy.
This is probably the easiest/safest way to achieve Party God Mode purely within the confines of the base game mechanics and without using Strand of Favor at all .