Guide to Playing Grade 8 ABRSM Dominant and Diminished Seventh Arpeggios
I find the main problem with playing arpeggios with a seventh in them - either diminished or dominant, is getting used to using the 4th finger. Every arpeggios learnt for Grade 5, (that is all of them!), only has three notes in each octave.
Start with this Hanon exercise, one hand at a time, initially:
Then follow this Guide to Playing Diminished Sevenths:
Finally, tackle the Dominant Sevenths here: