As I'm trying to get faster at these techniques, I always look at what the time constraints were for any particular shot. For this one they included researching tutorials and examples (this can often be the most time consuming part!), finding a decent freeware spine model; fine-tuning the position of the alpha mask; trying out different variations of the fluid flow; baking at a half-decent fluid resolution. Rendering was surprisingly quick but there aren't many objects in the scene apart from the spine and the fluid mesh.
For any composite effect to work you need to have some kind of interaction between the live action and virtual elements. Examples include lighting, shadows, foreground objects, reflections or physical interactions - in this shot the fluid is partially obstructed by some of the leaves on the ground.