Before hiring a lawyer, ask yourself if you stand a chance in this legal battle. Because, according to what I have seen in many standard contracts, a clause will state "employee is free to leave the position with xyz consultancy at any time." And, another paper will mention something along the line, "employee agrees to not take the job directly from the client before AA months is over with us."
If the consultancy is suing you, I am guessing they know they can win. So, unless you have any legal proof of otherwise, I suggest you to not fight the case, and simply do a out-of-court settlement. It will save your time, money, and mental pressure.