Technically, if Erza just wanted to win, the smartest thing would have been to challenge 51 monsters. That means that the next person, at best, can only get 49 points, so Erza would still have them beat! :)
As you realized, you misunderstood the points thing. There were 100 points up for grabs WITHIN the challenge. Those points were gonna be used to rank the teams based on the usual 10 to 0 point scale from previous challenges! :D
Cana is very powerful, it's just that the type of magic she uses isn't the most effective. Lucy has the same problem. She is clearly very powerful, but the spirits have many problems when it comes to battle :D