How powerful can a 20th-level Wizard make a 1st-level Fighter without allowing him to realize it is the Wizard’s doing?

A 20th level Wizard makes a bet with another 20th level Wizard – he has to make his grandson succeed on a difficult mission. Without allowing his grandson (a first level Fighter) or his grandson’s compatriots (a first level Rogue, Bard, and Druid) to realize that the Wizard is secretly aiding them, or doing anything to defeat their foes directly, or helping anyone but his grandson (these are all terms of the bet).

To clarify –

  • This Wizard can cast spells on his grandson, only. He can’t Dominate the monsters or Disintegrate locked doors.
  • His grandson has to defeat the challenges (the Wizard can’t even weaken them).
  • Neither his grandson, or any of his party members, can realize or even suspect that someone is aiding them.

Luckily the grandson and his entire party not only lack Spellcraft but also Knowledge: Arcana. However they don’t lack basic induction – suddenly being able to fly or shoot fireballs is likely to raise their suspicion. A magic sword left leaning casually against a door would certainly count as a fail.

So given this situation, with a level 20 Wizard who is unafraid to spend resources to win this bet with his buddy (xp, magic items, scrolls, favours) what can he do to make his grandson’s party succeed at an adventure that may have CR 5-6 encounters or worse?

For the sake of this example, assume the wizard has access to whatever feats, spells, or wealth he needs to achieve this difficult task – he’s using optimization tricks to rebuild or has his own time-slowed demiplane or whatever, he’s a Tricky Wizard and not just a fireball-slinger.