Shadow Field in Duels

Wanted to make sure I wasn't cheating other players when I have my archon in a duel with the bystanders cheering him on rule. If I roll a one on an armor save, I can reroll that for having 5+ bystanders and not short out the shadow field, right? I treated the one as something that never happened because of the reroll, but I hope that's correct.

Correct - it's a reroll just like any other

Yup you done good.
Who'd you fight?
With what?
Did you win?

Excellent, thanks guys. People tend to perceive rules (often unintentionally) in ways that are advantageous to themselves, so I didn't want to find out I was being biased.

I fought against a khorne daemon prince wielding an axe of blinding fury with my archon wielding an agonizer and shadowfield and supported by 5 wyches. My razorwing jetfighter heroically caused 1 wound, the ground caused a second, my archon got one through in the 2nd round of combat, and his own daemon weapon finished him off. Twas a sad end for a mighty warrior.

BOOM!
Well done, sirrah!
I've played in situations like that before, and DPs always feel invincible until that last wound shows itself. (My other army is CSM). And I've managed to have Typhus kill himself before as well.
Trick is not to use Daemon Weapons once they're down that low.

But anyway, you won CC vs a Daemon Prince and that's something to be proud of. Just be happy it wasn't a Tzeentch prince with Black Mace.

I've got a thousand sons army and playing against a friend with a nurgle army, we once had both of our generals sucked into the warp in the same turn. My Ahriman to a perils and his daemon prince to a bloodthirsty daemon weapon. One thing I like about my Dark Eldar, they don't go around killing themselves and turning into spawns.

Isn't there a rule somewhere about having to use the strongest weapon you have though? Or am I thinking of something in fantasy?

That's fantasy: if a model has a magic weapon, they must use it because "ooh, so pretty". In 40k, player chooses what weapon to attack with.