Here's a common affirmative defense: Plaintiff’s claims are barred in whole or in part because Plaintiff breached its contract with Defendant.

This means you had made a contract with the person suing you they didn't keep their end of the bargain. 

This defense is backed up by Federal Rules of Civil Procedure Rule 8(c)(1).

This is one of the defenses you can make in the SoloSuit Answer.