What is an ‘appropriate number’ of ‘significant foes’ for the purposes of Never Conquered, Never Feared?

So, I’m giving the players in my campaign a free Story Feat because I think they’re cool from a hand-curated list of choices that they and I feel are appropriate to their characters backgrounds and their motivations.

One of my players has chosen the feat ‘Never Conquered, Never Feared’ from the AP War of the Crown, which states as its goal:

You must individually slay an appropriate number of significant foes in succession, without retreating or withdrawing from a fight.

I’m trying to figure out exactly how to interpret this and would welcome some other peoples’ opinions. Looking at the rules for story feats an ‘Appropriate Number’ of foes is defined as:

These are either creatures whose individual CRs add up to 20, or creatures whose individual CRs add up to 5 times your character level, whichever is greater.

But these rules also define a ‘Challenging Foe’ as:

This is a foe or group of foes with a total CR of 10 or a CR of 3 plus your character level, whichever is higher.

However, the feat’s wording does not state ‘Challenging Foes’ but ‘significant foes’, so can I assume the definition of a Challenging Foe does not apply?

Also, would I be correct in assuming that ‘individually slay’ means ‘by yourself, with no help’?