ב"ה
Im fairly certain the that the math for the average crit damage a paladin can do with the opportunity of rerolling is pretty complex.
Not against resilient creatures. Resilient creatures and creatures without armor are always easy.
On a standard roll you have probability on 1/6 to get 1 crit, 1/6 to get 2 crit and 2/3 to get nothing, so you get 1/2 damage from each die, to a total of 1/2 the number of dice you roll.
Since we assume the paladin tries to maximize the average damage, the paladin wouldn't reroll the 1 critical or 2 critical (since 1 and 2 are greater than 1/2) but only the dice thast didn't roll any critical damage. So we have 1/6 chance to get 2 crit on the first roll, 1/6 to get 1 crit on the first roll, and 2/3 chance to reroll.
The rerolled dice are simple rolled, so they have an average damage of 1/2, for a total of:
1/6*2 + 1/6*1 + 2/3*1/2 = 5/6.
(2 crit on first roll) (1 crit on first roll) (rerolling) (total)
Each die give an avarage damage of 5/6, so the total is 5/6 times the number of dice.
Paladin reroll is once per round.
So the choice when to reroll with challenge and when to reroll with akiro's favor could boost the average damage a lot. it really changes th
The worst case is if we assume the paladin use challenge rerolls on the non-crits of the first attack regardless of the amount of crits rolled, and akiro's favor isn't used would be 9+1/6 for the first attack and 5+1/2 for the second attack for a total of 14+2/3 (which is almost enough). Since the paladin had akiro's favor, and the choice whether to hold the challenge rerolls for the second attack (if he already got many crits), it would go higher. And with eye for an eye it is again most likely that the elemental would die anyway.