One of Phantasy Star's special features is the ability to combine spells. Here is a list of all known combinations. Unless otherwise noted, you can use the stronger or weaker version of a spell component (i.e., Foi instead of Nafoi.)

SPELL COMBINATION

Fire Storm Zan + Foi, Foi + Hewn, Flaeli + Zan, Flaeli + Hewn
Blizzard Wat + Zan, Wat + Hewn
Silent Wave Phonon + Airslash
Triblaster Wat + Foi + Tsu
You cannot use the more powerful versions of these spells for this combo.
Grand Cross Crosscut + Efess
Shooting Star Burstroc + Nafoi
Paradin Blow Rayblade + Astral
Lethal Image Death + Illusion
Black Hole Negatis + Nagra
Circuit Breaker Hijammer + Tandle
Only works on mechanical creatures.
Holocaust Savol + Diem
Only works on biological creatures.
VOL cannot be used for this combo.
Destruction Deban + Megid + Legeon + Posibolt
This one must be done in the order stated.
Purify Light Holyword + Efess
Conduct Thunder Nawat + Tandle