Never wholesale copy a copyrighted work without adhering to its license: if no license is supplied, all rights are reserved and only fair use/dealing applies, meaning you can't re-host the work elsewhere without express permission of the copyright holder.
Ideally, as an exercise in fair use, you should always excerpt1 the relevant part of the guide when writing your answer in addition to linking to the full guide. This way if link rot occurs, the information is still available within the context of the larger answer.
But since that didn't happen and the guide is no longer available, the reference to the guide should be removed. It doesn't look like your answer significantly changes if you remove the following line:
Crosscounter created a quite good free guide that explains everything.
Of course, if someone wants to ask the admin of the site to re-host the guide or make it available under terms that let you re-host it elsewhere, awesome: add back the reference when it becomes available again. An alternative would be to find another guide that contains a similar explanation to what you wanted to reference. Just remember to include an excerpt if either happen.
Note 1: there is no standard or accepted amount for what constitutes fair use and what amounts to wholesale copying, but a rule of thumb I use is that a) the excerpt should not be the main thrust of the larger work (your answer), and b) the excerpt should not completely obviate the value of the guide.
