They are both correct but the meanings differ. I'll meet you there for lunch = lunch is the reason for the meeting, so we assume that 'there' is a place that sells food and that you and the other person will eat lunch together. I'll meet you there at lunch = lunch is a time. Lunchtime would be more common but, because it follows 'at', 'lunch would also be assumed to be a time. So we do not know why you are meeting and we do not know what kind of place 'there' is. We just know you are meeting at whatever time lunch is.