The Arthur Vandelay Disagreement Hierarchy

[VeniteAdoremus]

I usually start in the orange and then work my way up to purple or grey

"Your source smells funny, doesn't have any knowledge on the subject, is mistaken here, here, and here because of this and this, and by the way, your main thesis is [i]obviously[/i] wrong, cf. X et al, Y and Z."

In competitions, f you go slightly ad hominem at the other's sources, and use good arguments straight afterwards, chances are that he'll still be bristling at the personal attack and not actually listen to your arguments. Then he'll come back to the ad hominem, and you can patiently inquire why he doesn't want to talk about the arguments you just used. Win.