There is a nice summary of combinations on phantasy-star.net, stating 14:
Ever since PSIV was, there have been
claims that there are more than the
fourteen combos listed below.
Instruction books and hintbooks claim
that there are fifteen or more.
However, in all the years the game has
been out, no legitimate combo has been
published on the net, other than the
ones below.
Black Hole - Negatis /-\
(Gra,Gigra,Nagra)
Blizzard - (Wat,Giwat,Nawat) /-\
(Zan,Gizan,Nazan) or (Wat,Giwat,Nawat)
/-\ Hewn
Circuit Break - Hijammer /-\ Tandle
Conduct Thunder - (Wat,Giwat,Nawat) +
Tandle
Destruction - Deban + Megid + Legeon +
Positron Bolt
Firestorm - (Zan,Gizan,Nazan) /-\
(Foi,Gifoi,Nafoi) or (Foi,Gifoi,Nafoi)
/-\ Hewn or (Zan,Gizan,Nazan) /-\
Flaeli or Flaeli /-\ Hewn
Grand Cross - Crosscut /-\ Efess
Holocaust - Savol /-\ Diem
Lethal Image - Death + Illusion
Paladin Blow - Rayblade /-\ Astral
Purify Light - Holyword /-\ Efess
Shooting Star - Burst Rocket +
(Foi,Gifoi,Nafoi) or Burst Rocket +
Flaeli
Silent Wave - Phonomezer /-\ Airslash
Triblaster - Wat /-\ Foi /-\ Tsu
where () means any skill or technique
within may be used; /-\ means the
order of the skills or techniques does
not matter; and
+ means the skills or techniues must be performed in the specified order;
To answer the second part of your question, the most powerful combination is Destruction, as described in this article.
