Well Ling did took on Wrath with Lan Fan on his back and was able to dodge the Fuhrer's attacks....
He took him on while he was Greedling/with Greed's powers while backed by Lan Fan, Fu, and an army of Briggs soldiers including Buccaneer. And he was getting stomped by him easily. Alone, he stood no chance. Without Greed's help, he'd get one-shotted. It literally took a Bradley who had to be blinded from someone as brutal as Buccaneer to at least be scratched (and that took a suicide attempt from Fuu). Even as Bradley was wounded from that attack and an enraged Ling leapt at him, he still can't beat Bradley alone.
Also Scar couldn't even handle a half dead blood lusted Fuhrer...
He kinda did.... and he did it alone.
But I'm not basing this fight off of their performance against Wrath and the conditions of it, seeing as Ling was heavily outclassed by Wrath and Scar "defeated" a wounded and strained Bradley out of pure luck.
Speed aside, if this is a battle between normal Ling and Scar, Scar has a great physical/strength advantage. If he tags Ling, it's practically over for him. And having the reconstruction alchemy allows him to use the surroundings against Ling too. One carbon sword is not enough for him to win this fight.
Ling was still a normal human when he first faced Wrath and Gluttony... and h still manage to deal with them
Also Greed was doing most of the fight, I don't see Ling getting killed by Wrath easily...
Also Ling is pretty strong... he was able to lift Gluttony.... and he is a better h2h combatant than Scar, And Scar had difficulties fighting a half dead Fuhrer.