It really depends on the nature of the spellcaster.
In very general terms?
A Lawful Evil opponent would use the system against the heroes, twisting the knife by turning the system of justice against them. False witnesses, illusions, plots, insinuations would all serve his goal. He woudl destroy their homes, their lives and their roles in society. Tax them, burn their homes, dissolve their social structures...and make sure they understand that the law applies inequally.
A Neutral Evil wizard of greater power would hound them, make them pariahs, and undermine their goals. He would cheat them of victory, turning their successes to ashes in their mouths. He would be unpredictable, adhering to some obscure rules one day, and violating them the next. He would sabotage their efforts, rob them of their confidence and violate their feelings of security and trust. All of this would be done with a distinct lack of passion or heat. Not calculating like the LE villain or gleefully viscious like the CE villain, he merely forwards his own ends.
A Chaotic Evil villian would leave a path of destruction and sow fear all around them. He might revisit past successes and reverse all the good they did, and make sure they knew who had done it, why, and provide revolting details. He would strike without warning, and with malicious glee. He would be random and unpredictable, seeking not just to defeat the heroes, but to crush their spirit, and to prolong their suffering. Where the LE villain would use hostages as a bargaining chip and the NE villian would use them as a tool, the CE villian would use them as a weapon - torturing them, making copies of them or even using them as spell components.
There are lots and lots of ways for a powerful spellcaster to undermine the players...but make sure to do so in moderation and to make sure he gets a serious on-screen comeuppance, or it will be very unsatisfying to the players. Remember to give them little victories along the way.