Iron shot, definitively. Very strong, and very effective against OoF.
Forget Bolt of Magma (3 schools, you don't really need fire school). With only 2 spell (Lee rapid deconstruction and Iron shot) you can win the game. Don't spread yourself with a lot of schools, and focus instead on fewer of them.
I won with a GrEE and finished the 3 runes game with these :
Skills:
* Level 21.4 Fighting
- Level 12.0 Staves
- Level 19.1 Armour
- Level 6.0 Dodging
+ Level 21.4 Shields
- Level 0.2 Unarmed Combat
- Level 18.0 Spellcasting
+ Level 22.0 Conjurations
- Level 3.0 Hexes
- Level 4.0 Charms
- Level 3.1 Summonings
- Level 3.0 Necromancy
- Level 5.3 Transmutations
O Level 27 Earth Magic
- Level 17.0 Invocations
- Level 16.0 Evocations
I had very few spells, too :
a - Summon Butterflies Summ ######.. 14% 1 None
b - Stone Arrow Conj/Erth ###### 0% 3 None
c - Petrify Tmut/Erth ######## 1% 4 None
d - Regeneration Chrm/Necr ######.... 34% 3 None
e - Lee's Rapid Deconstr Erth #########. 1% 5 None
f - Passwall Tmut/Erth #########. 1% 2 None
g - Spellforged Servitor Conj/Summ ########.. 93% 7 #......(never used it, though, found it at the end of the game).
h - Leda's Liquefaction Hex/Erth ########.. 1% 4 None
i - Swiftness Chrm/Air ######.. 32% 2 None
j - Iron Shot Conj/Erth #########. 1% 6 None
k - Shatter Erth #########. 77% 9 ######. (never used it, though, found it at the end of the game).
I had quite high failure rate because i found a +8 orange crystal plate armour which, combined with a staff of earth, gave me actually a lvl 29 in earth and 65 AC with 33 SH.
You can 2 or 3 shot even the most powerful foes (like Ancien Liches) with Iron shot. Don't bother with Bolt of Cold or Bolt of Magma ! (which is, in the latter case, a bad spell anyway)
Petrify can help with 2 things : Slow enemies to run if necessary, or petrify them to make explose them. But the second one is practical. You have to wait for the enemy to be stoned for full explosion, and his friends may have moved a lot in the same time.
About order, i suggest you finish dungeon, or at least until 14 before going to orc. Orc can be sometimes surprisingly hard, and you can't run so easily like in Dungeon. I suggest you finish Dungeon, therefore, and be quite careful here.
(sorry for bad english, it's not my main language)