This can be done in Practice Mode.
The achievement description is misleading after the Season 3 update. Rather than cursing an opponent with Searing Skull, you specifically need to land an Instinct Curse, which is the shadow variant.
Spinal uses skulls as part of his special moves; activating Instinct Mode will give you the maximum amount of skulls and constantly regenerate them. Pause, hit
twice to get to Practice Options, set Instinct Meter to Infinite or Refill.
Activate Instinct Mode and facing right, use
with default controls, or any two punches. Once it hits, achievement unlocked.