Just so folks know, there are tools that will decompose queries and make nice little pictures out of them. With something like this, you'd have to use it just to get started.

Once you've visually decomposed it, you'd physically decompose it by splitting it inside-out. Then proceed to understand and debug inside-outwards.

Not fun, but not impossible. Just a huge pain in the ass. Making it more fun would be a database with bad RI, nulls, and duplicate data all over the place. Don't get me wrong -- from looking at the image it definitely looks like aspirin will be required. :)