Oh, I should have mentioned diminishing returns on other stats, since you asked. I started writing about INT and then I forgot.... xD Magic peaks at around 40-45, and hits greatly diminishing returns at 50, so never go above 50. Endurance should never, ever go above 40 (equip burden is unnecessary with light armour, and stamina caps at 40), and since you're a mage, you don't need as much endurance as a melee fighter so it's okay to have lower END. But that's up to you if you feel comfortable fighting with more or less stamina.
Definitely get Soul Ray. It's like Soul Arrow on megasteroids. Spice is cheap, especially from the singing lady in Latria, and the spell is worth it.
Freke and Yuria both teach powerful spells. Some boss souls can be used by both, so it's a matter of choice which of the two you'll pick... but fret not, you can always get the other in NG+. Here are noteworthy boss souls for offensive spells:
- Fool's Idol: Soul Ray (Freke). No-brainer, it can't be used for anything else. Your main ranged attack, most likely. Get it!
- Armour Spider: Ignite (Freke) or Firespray (Yuria). Ignite is like Great Combustion in Dark Souls. Very powerful, but very close-range and requires 2 spell slots. It's not that great in PvE but has some really nice PvP uses. Firespray is ranged, and fairly weak, but it can be cast repeatedly by holding the button at low MP cost and you can move while you cast, so it's definitely useful in both PvE and PvP, especially against annoying flying enemies.
- Dragon God: Firestorm (Freke) or Fireball (Yuria). Firestorm is a medium/short-ranged nuke. With the right gear and stats it can OHKO some bosses, it's ridiculously OP. Costs 3 spell slots though! Pretty bad in PvP because it leaves you vulnerable to backstabs, but in PvE it's easy mode. Fireball is ranged and very powerful, extremely good in PvP when used properly, and also very good in PvE. Costs 2 spell slots.
- Old Monk : Homing Soul Arrow (Freke) or get the Insanity Catalyst (Blacksmith Ed). Equivalent of Homing Soulmass/Crystal Soulmass. Very strong at full power. Terrible in PvP but pretty good for PvE. The Insanity Catalyst is the strongest in the game but reduces your MP by half. Since spice is cheap, it's still fairly useful with high enough INT.
So you'll need to decide where to spend the souls for the Old Monk, the Dragon God, and the Armour Spider. All of those choices are valid, it's a matter of preference.
Warding is a defensive spell and does not scale with your magic power. It's quite useful, drastically reduces all physical damage taken for about 40 seconds. I prefer Second Chance (you can't use both at once, though you can always cast Warding when Second Chance triggers since the casting time is quick) but it can be a good support spell before getting SC. Worth learning for sure, and memorizing for when you're struggling to stay alive.
Flame Toss is pretty bad, only use early in the game until you get the better spells.
