Any twisting training will condition the area if needed, boxing, weight plate twists (bend at the knees), swimming (crawl) will be effective. In holistic terms the whole region should be stabilised to lift and support the spine- drawing the stomach, obliques and lumbar support muscles into their true original alignment. I can only agree with the aussie gent- Can't see what needs "slimming" on that picture...
honestly looks like form is there to me, as far as strengthening and going for the 6 pack Work compound excercises, stay away from side bends and weighted twists.