The advanced insta-kill saves about 1.5s over the normal beginner strat.
Beginner - No Quick Kill
- Jump dive and rollout while moving left.
- Hold up and chase Shadow Mario up the ramp while spraying until he goes down
Alternative to the jump dive and rollout, you can jump in place and move slightly left, then just run forward and start spraying when Shadow Mario is triggered.
Advanced - Insta-Kill
- Jump dive and rollout, then dive towards Shadow Mario
- Briefly hold down left to slow down and align yourself behind Shadow Mario
- Press digital R and A to spam spray while performing a dive recovery to defeat Shadow Mario
There are two alterntives that are in between the beginner/advanced strats in terms of speed which involve triggering Shadow Mario early. They can be seen here and here.