1) Asperse 2) Discredit 3) Smear 1) Assail with arguments 2) Assail with words 3) Attack as false 4) Attack as false or wrong 5) Attack by argument 6) Attack verbally 7) Call into question 8) Cast doubt 9) Cast doubt upon 10) Challenge 11) Challenge as false 12) Challenge in a way 13) Challenge in argument 14) French word used in English 
to oppose; to attack by words or arguments
• (v. t.) To attack by words or arguments; to contradict; to assail; to call in question; to make insinuations against; to gainsay; to oppose.
Im·pugn' transitive verb [ imperfect & past participle Impugned ; present participle & verbal noun Impugning .] [ Middle English impugnen , French impugner , from Latin impugnare ; in on, against + pugna...
[v] - attack as false or wrong
impugn 1. To suggest that something cannot be trusted, relied on, or respected. 2. To attack as false, questionable, or wrong.
verb attack as false or wrong
to challenge as false (another's statements, motives, etc.); cast doubt upon. · to assail (a person) by words or arguments; vilify. · to attack (a person) physically.