Do you lose damage from sand/hail first or do you heal from leftovers first?
Gliscor restored hp using leftovers! Gliscor was buffeted by hail!
or
Gliscor was buffeted by hail! Gliscor restored hp using leftovers!
What about for poison heal?
>This healing occurs after damage from sandstorm or hail, but before damage from poison, burn, Nightmare, or Leech Seed.
Source
You always take damage first. So hail/sandstorm would damage Gliscor, then he would restore health from Leftovers. Then again, Gliscor doesn't take hits from Sandstorm anyway.
Source: Experience.