A Kangaroo Skill is simply three Air, of any degree, skills in quick succession.
One place to get these done is just south of the Gold Mine. There is a collection of mounds that you can jump all bunched together.
However, I prefer to go all of the way out west, south of the Pink Lakes. If you do it here the jumps combined with both wreckage and drift skills, as well as all of the combinations, will allow you to knock this out as well:
Just stop every time you hit an Ultimate Skill chain and bank it.