This works for everything except one big case. Project with 90% of work done but the last 10% is the one holding the project to be released because it's a hard

This works for everything except one big case. Project with 90% of work done but the last 10% is the one holding the project to be released because it's a hard but core functionality of it and all previous programmers didn't touch it/knew how and the customer is adamant about it. So I get the ugliest part with the high expectation from the client to be finished fast. I usually avoid this type of projects as well, but in few cases where I did it I found the only working solution is the rubber duck one. Usually I use my wife (non-technical person) as the rubber duck and I explain to her, while she is in the kitchen cooking, all the steps I need to do in order to complete the last 10%. After the 2 hours filling her head with technical details, to which she pays absolutely 0 attention to it, I can do it. So, tldr; -> rubber duck technique.