Shulk has 4 guaranteed set-ups that lead into footstools: U-air1, U-air2, N-air, and F-air. Although you can follow-up with a footstool in any art, the most useful and reliable art to footstool in is Jump Art
Typically speaking, the most common opportunities for Shulk to get a footstool combo to connect on an opponent are by reading an opponent's defensive/offensive option, such as:
- Shieldgrab or dash-grab approach
- "Get-off-me" tilts (e.g., Marth's f-tilt or Fox's U-tilt)
The other 10% is via random U-air approaches (as a rare mix-up only though) and hard punishes when the opponent goes for a hard Smash attack read or an Up-B OoS (after they wiff) .
This set-up is one of the hardest out of the four because of the hard-to-hit-with nature of U-air1, but it's unique in that it will allow Shulk to connect with a guaranteed footstool when the opponent is at any percent, whether it be 0%, 100%, or 999%. This is because U-Air1 has set knockback, with the only thing ever affecting its knockback being Shulk's Rage (which doesn't change things significantly).
This one can be the safest footstool set-up to go for, since falling U-air has the potential to be safe on shield if you cross the opponent up. It also deals the most damage, and has the largest window for error when it comes to landing your footstool since it delivers a lot of hitstun. Depending on the character, U-air2 will reliably allow you to connect with a footstool between 0-30%. Eventually, the U-air will launch them too high for Shulk to get a footstool off it
For N-air → Footstool to work, you must hit with the N-Air when Shulk is as close to the ground as possible, going as far 'into' the opponent as possible without crossing them up (AKA knocking them behind Shulk). It is impossible to connect with an aerial footstool on fast-fallers like Fox and Falco, but floaty and short characters such as Lucas, Luigi, and Kirby are very susceptible to this footstool set-up.
GetShulked is still labbing this one, but it seems to connect true into a footstool between 20% and 40% for all DI except down and away for fast-fallers
After successfully footstooling your opponent, often the most optimal follow up is definitely to go for a reset with dair (sometimes incorrectly referred to as a "jab lock").
Follow your opponent's footstool DI left or right by delaying your fast fall D-air as long as possible.