Above all, be on their side and not against them. This is the crucial non-quantifiable element to a good handling of a guest complaint. Do not phrase it as an us vs. them situation. People skills help. Strategies include matching your mood to theirs i.e., down in the dumps and angry and gradually elevating the mood toward positivity, so that they come with you quite naturally and indeed unconsciously.
Some guests shout.
If you are soft, the guest usually softens up his tone again naturally and unconsciously. This can cause friction, and handling this kind of complaint is the most tricky. The trick is to maneuver him into the mindset you have, but without arguing or contradicting. This takes practice. Thus, the final part of this very helpful class is some practice.