Too physically weak to do the job (2 Cor. 10:10)
Underhanded, cunning (2 Cor. 4:2 RSV)
Tampering with God's word (2 Cor. 4:2 RSV)
Not preaching according to the sanction of the Lord Jesus, but inventing things for himself (in the context of Gentile liberty, Gal. 1:1).
Preaching himself as the saviour, not Christ (2 Cor. 4:5)
Commending himself, showing himself to be so spiritually strong (2 Cor. 3:1)
Trying to build up his own self-image with his listeners as he preached the Gospel (2 Cor. 4:5)
Trying to domineer over his brethren (2 Cor. 1:24; 8:8 Gk.)
Mentally unstable (2 Cor. 5:13)
Causing others to stumble (2 Cor. 6:3)
An imposter (2 Cor. 6:8- in the context, Paul is saying that the fact he is so maligned is a kind of proof that he really is a genuine worker for the Lord!).
Wronging, corrupting, financially defrauding brethren (2 Cor. 7:2)
Demanding so much money from others that they would become impoverished themselves (2 Cor. 8:13,14 J.B. Phillips)
But not a real apostle, seeing that if he was then he would do as the Lord had bidden and receive “hire” for being a “labourer”; if he was worthy, he would have accepted it. The fact he didn’t showed he wasn’t a hard labourer. This was so untrue. It's a real cruel example of slander in the church.
He only threatened ecclesial discipline but never did anything in practice- he was all talk and no do (2 Cor. 10:1-6)
What he wrote was in his letters was a contradiction of the person he was in practice (2 Cor. 1:13)
He kept changing his mind over important issues (2 Cor. 1:17-19)
They were offended that Paul didn't take money from them (2 Cor. 11:7 RSV), and yet also grudged giving money for the Jerusalem Poor Fund because the Corinthian church slandered Paul that he claimed he was only trying to get the money for himself.
Crafty and a liar, not opening his heart to his brethren (2 Cor. 12:16 cp. 6:11)
Preaching that we can be immoral because God's grace will cover us (Rom. 3:8)
Preached in order to get money and have relationships with women (1 Thess. 2:3-12)
Still secretly preached that circumcision was vital for salvation (Gal. 5:11).
Note: If you can imagine where Paul might have used quotation marks, this helps to reveal certain phrases which he was probably quoting from their claims. Most of the above slander in the church was from just one ecclesia (Corinth): one can be certain that there were many other such slanders. He very gently tried to encourage them to raise money for the poor saints in Jerusalem, in order to justify his boasting of them. Finally, after all his spiritual diplomacy in raising the fund, he had to ask the Romans to pray with him that the Jerusalem ecclesia would accept it (Rom. 15:31). Presumably they didn't want to accept help from Gentile converts whom they despised. And if they didn't accept it, then Paul would look as if he had got them to raise the money just to give to him. There must have been times when he thought of quitting the Christian community because of slander in the church. Paul was not a larger than life figure in the eyes of the early church. They didn't see him as we do. The harder he worked, the more he was slandered, and the more painfully. Every genuinely hard working brother in the Christian community has suffered the same slander in the church.