Hitting your target, parrying, critical hits, double hits, and elemental status
are controlled by random chance. On the one hand, this adds depth through
management of risks and rewards, and it forces unexpected events on the players.
On the other hand, it can quickly make combats feel unfair (e.g. good tactics
failing due to bad luck) or infuriate players (e.g. missing a 95% chance shot in
XCOM).

To mitigate the drawbacks, TO should balance the impact of luck.

During a battle, each players has a Luck meter. Luck in between -200 and 200. It
starts at 0.

When a player makes a roll with X% chance, the result is as follows:
Roll = rand(100);
IsSuccess = ((X + Luck) >= Roll);
if (IsSuccess)
{
   if (X > 50)
   {
      // It was likely to succeed, pay just what you used.
      MissingPoints = max(0, (Roll - X))
      Luck' = (Luck - MissingPoints)
   }
   else if (X =< 50)
   {
      // It was unlikely to succeed, pay a lot.
      Cost = (55 - X) // How far was it from being reasonable?
      Luck' = (Luck - Cost)
   }
}
else
{
   // Failure due to bad roll.
   if (X >= 50)
   {
      // It was likely to succeed, you're owed a lot.
      OwedPoints = (X - 45)
      Luck' = (Luck + OwedPoints)
   }
}

For example:

A/ The character of a player with 15 Luck attempts a 10% chance parry:
if (Roll <= 25)
{
   // Unlikely Success.
   Cost = (55 - 10) = 45
   Luck' = 15 - 45 = -30
}
else if (Roll > 25)
{
  // Expected Failure.
}

B/ The character of a player with 20 Luck attempts a 45% chance parry:
if (Roll <= 60)
{
   // Unlikely Success.
   Cost = (55 - 45) = 10
   Luck' = 20 - 10 = 10
}
else if (Roll > 60)
{
  // Expected Failure.
}

D/ The character of a player with 20 Luck attempts a 55% chance parry:
if (Roll <= 70)
{
   // Likely Success.
   UsedUpLuck = max(0, (55 - Roll)) = [0, 20]
   Luck' = 20 - UsedUpLuck = [0, 20]
}
else if (Roll > 70)
{
  // Unlucky dice roll.
  OwedPoints = (55 - 45) = 10
  Luck' = 20 + 10 = 30
}

D/ The character of a player with -35 Luck attempts a 15% chance parry:
No roll required. This fails. No luck update.

E/ The character of a player with -35 Luck attempts a 75% chance parry:
if (Roll <= 40)
{
   // Without the negative luck, it was likely to succeed
   // Luck clearly didn't help, so none is to be payed.
}
else if (Roll > 40)
{
   // If not for that negative luck, you were likely to make it.
   // You're owed some luck back.
   OwedPoints = (75 - 45) = 30
   Luck' = -35 + 30 = -5
}