How many casters?
Some people like all their eggs in the same basket, some other like to spread the love around. I'm personally in the middle. A lvl4 and a lvl1(2) is usually my favourite combo, allowing two lore to be chosen and increasing your change to get a good spell for your matchup (in case you're not tailoring your list). The extra caster also gives you an extra chance at channelling, which should theorically bring 2 extra dice / games. A second caster would be the first time to be removed if i ever need pts, but otherwise, its always a good idea to bring an extra one. When it comes to choosing which lore for the lvl1(2) one, its usually a good idea to go for a lore with a good signature spell like Beast, Shadow or Heaven.
How to dispel and Magic Defense.
When thinking about magic phase, its not all about casting, its also about dispelling. If you do not manage your dispel dice correctly, you may end up with a Dreaded 13th, or an Infernal Gateway whiping half your army.
Obviously, there's no strategy that can guarantee you an easy dispel phase, and each army/lore have different parameters that can modify the result. For example, if you are playing against a Goblin army, you may have to consider dispelling worthless spell, just to avoid him stealing you a dispel dice, and weakening your magic defense.
First thing to know, is your ennemy, and which spell he has. You have to keep it in mind at all time, to make sure you do not let a powerful spell through expecting something else that will just never happen (because of range or LoS). Knowing what can be cast at all time is a great help in knowing what to dispel.
Second thing to consider, is what spell you can survive if it goes through, and which you can't. For example, if you ennemy is casting a buff on one of his unit, but the unit is not in combat and you have no plan to charge him next turn, it might be a good idea to let that spell through. Same goes for debuff. If you think it won't affect your overall plan, no reason to throw dispel dice at it.
Third, what is a game breaker and what isnt? a 2D6 S4 magic missile on your unit will hurt, but considering its on average 7 hits, and 5 wounds on T3, is it really that bad?
How to cast and managing Risk.
Each magic phase is different, each ennemy is different, and each army is different. So how to adjust your playstyle? With managing Risk. Most of the time, you'll have between 4 and 7 spells available, and roughly 7 power dice.
There's two way to approach this phase. 1st one is the obvious; throwing 6 dice at a spell that will obliterate one of your opponent unit, with the risk following that tactic, and the second, is to force your opponent to throw dispel dice. Except in very rare occasion, i'll never throw 6 dice. I even rarely throw more than 3 dice at a spell.
When it come to casting, a good rule of thumbs is to consider that each power dice should be a 3 or better. So if your lvl4 want to cast a spell with a 11 requirement (including your caster), use 3 dice. Your chance of success are roughly 90%, with only 7% of miscast. Going with 2 dice could work too, but its only 58% success, so its better used when you have a limited power pool, or not really anything worth risking a miscast.
Here's a table to keep in mind at all time ;
What to cast and when.
Another very important part of your magic phase, is deciding which spell to cast first. If you go with your most important spell (like regen for Ogre, Mindrazor for Shadow, or Dweller Below for Life) first, your opponent will obviously try to dispel it with all his dice. If you decide to keep him for later, your opponent will most likely keep his dispel dice to dispel it. In both strategy, you should be able to cast one or two lesser spell without much problem. Going with smaller spell first might cause you to lose concentration on a bad roll, and just never manage to get your big spell off. Going the other way around risk you to either miscast or lose concentration.
My favourite approach (with my ogres) is to go for smaller spell first, trying to get as many through as possible. +1 Toughness, 2d6 magic missle, sometime a fear test, and then BAM! regen.
So there it is, how i approach each magic phase.