Copy and paste man, copy and paste. Do so in an irregular way, mix it up, but it saves time and no one will really notice if you do it randomly enough. If you got some good wall pieces arranged that look nice, do exploit it. As someone who's been working on a levelpack with lots of levels with the official tilesets - for literally years - I can assure you that it did take me a good while to find everything these tilesets have to offer and how they work, and it is a pain in the ass, but the results can be really pretty - still not enough custom tilesets look as good as Stadler's. Just rest assured you're not the only one struggling but you can get the hang of it.

also, hope you're ready for trying the hell entrance tileset, because it's one hell of a ride