How do you quickly learn to fix something that you have little/no experience repairing?

My boss likes to tell customers that we are able to fix things that i, his head technician, have little or no experience fixing. I mainly specialize in high-volume digital consumer electronics such as phones, computers, game systems and the like, but my boss likes to take in stuff such as DSLRs, home theater systems, commercial high-voltage audio amplifiers, professional grade analog audio equipment, and all other manner of things that i personally feel i have no business working on. These often take the form of niche devices that i will probably only see come into the store once and then never again, as opposed to the PlayStations and iPhones that we get in every day. He claims that he has experience fixing all of this stuff but that he wants me to try to "figure it out for myself," and half the time i end up breaking something even if i manage to find a guide to follow.

Does anyone have any advice on how to handle these sorts of situations? How do you quickly learn how to fix something when you've never worked on anything like it before? I'm not at electrical engineer and don't know how to reverse-engineer a device without any documentation, but often times the things I'm asked to repair don't have any documentation or guides available on how to fix them, and even when they do i generally lack the necessary experience due to never having worked on those things before.