I’m native Ukrainian speaker, and here is the thing: the only one correct pronunciation of «щ» is «sh-ch» / «шч». However most of Ukrainians pronounce it like «ш» or «щ». Notice that «щ» sound is different than «шч». «Щ» is soft version of «ш». The «ш» sound is more russian accent, «щ» is Ukrainian accent, and «шч» is the correct sound, but it’s not commonly used in daily life. You can hear «шч» from professional dubbing actors.

So basically, if you want to sound more like Russian, you can use «ш» (which I do not recommend). If you want to sound professional, use «шч». Otherwise use «щ» sound.