If the song reference works for you, that's good. I could never get that method to work for me. I found it too slow to recall the reference song and playback the interval in real time.
After months of trying to drill isolated intervals and getting nowhere, I had a lightbulb moment. I know the sounds of these intervals like the back of my hand and I just needed to put a name to them. I stopped telling myself it was hard and difficult and that eliminated a major roadblock. I started listening to songs and sometimes a certain chord progression would sound so familiar. I would sing the notes or look up the progression, and voila it was much quicker and so much more fun to internalize the sounds while putting a name to intervallic relationship. I started picking up patterns that had a certain sound and feeling to it, I could easily pick out the intervallic movement. 145 in any order are the easiest for me right now. I've played those patterns in real songs and can spot them. Musicademy & Steve Stine both have an ear training course that blows everyone else's is out of the water. They focus on real application in a song plus there is emphasis on humming sounds to internalize what you hear.
In a way what I did is not much different from the drills approach except my mind was more engaged & excited when I had to reverse engineer songs I liked then giving what I heard a name.