To reach Level 45, one of the requirements is to purify 100 Shadow Pokémon. This can become quite costly in terms of limited resources! To minimize the Stardust costs, it is best to focus exclusively on Shadow Pokémon that only require 1,000 Stardust for purification.
Use the following search string to find cheap Shadow Pokémon in your Pokémon Storage to purify (change "shadow" to the appropriate equivalent in your language):
shadow&10-22,25-26,35-36,39-42,74-76,129-130,161-174,218-219,223-224,261-269,276-277,283-284,293-295,304-306,316-317,320-321,325-326,333-334,339-340,396-402,412-414,420-421,504-510,519-521,527-528,540-542,546-550,556-558,587,602-604,659-663,819-820,580-581,590-591
Alternatively, you can also use the more explicit version (keep in mind this only works if you have your language set as English):
shadow&+Caterpie,+Weedle,+Pidgey,+Rattata,+Zubat,+Geodude,+Magikarp,+Ledyba,+Poochyena,+Whismur,+Aron,+Wailmer,+Spoink,+Barboach,+Starly,+Bidoof,+Patrat,+Purrloin,+Pidove,+Dwebble,+Ducklett,+Foongus