It is difficult, but not impossible. There are many things that help. My ex-girlfriend had several visas to Europe already which definitely helps. And her sister is a travel agent, so she had some connections to tour companies, which also helped. But the most important thing is showing you plan to return.
In a worst case scenario, the man can visit the girl once or twice. Apply for a k-visa and then apply for a tourist visa to travel before the k-visa is complete. If you can prove the relationship and support, it is possible to get the tourist visa for a visit and if the relationship does not work, the k-visa can be cancelled.
So, difficult, yes, impossible, no. If going for a visa, it should be as strong of an application as possible.