Whether or not that duty has been breached can be a question of law or fact depending on the circumstances. Malpractice in appellate law, for instance, is an area a layperson has no business deciding, according to many jurisdictions, and is solely a matter of law. Legal malpractice cases can be nipped in the bud by a sympathetic jurist. Expert testimony is, of course, generally required. When it comes to expert witnesses, the greyer the hair, and the longer the track record in the same area of law, the better.
Retired judges do nicely.
A past grievance or malpractice case against your expert is not necessarily a fatal flaw. Mistakes corrected graciously and honestly go a long way toward making the defendant who is resisting doing the same look worse by comparison. The toughest element in a legal malpractice case, however, is proximate cause, since it requires proving that the client, but for the negligence of the lawyer, would have prevailed in the underlying lawsuit. Proving a malpractice case does not require or contemplate that the jury and judge in the underlying suit be queried as to whether or not they would have decided the client's fate differently had the lawyer brought in the witness that would have made the case, for instance. The question is what the fact finder should have found if the case was properly prepared, or what could have been reasonably accomplished with the opposing party in terms of settlement had the lawyer not missed the deadline or lost the evidence.