!ask can Baron be affected by Ahri champion unit and/or Ahri legend

Yes for the Ahri Legend, no for the Ahri champion unit.

• Ahri Legend (Nine-Tailed Fox): Yes. "When an enemy unit attacks a battlefield you control, give it −1 Might" never chooses or targets — the attacking unit is programmatically selected by the trigger. Baron's "I can't be chosen by enemy spells and abilities" doesn't stop effects that don't choose (Rules 755–758). So if Baron attacks into a battlefield the Ahri player controls, he gets −1 Might this turn (min 1).
• Ahri champion unit (Ahri, Inquisitive): No. "Give an enemy unit here −2 Might" requires you to choose a unit, and choosing a unit is targeting (Rule 355.10). Baron is untargetable by enemy abilities, so he's not a legal choice — Ahri's controller must pick a different enemy unit at that battlefield, or the trigger does nothing if he's the only one.

Ahri, Alluring (the other champion) doesn't target at all — it just scores on Hold, so it never interacts with Baron.