Ok, total warhammer magic. First thing’s first.
Are you aware that doubleclicking a spell (not every spell but most, and you can check which spells in the spell browser in the top left of your UI) will overcast it, and overcasting it yields sometimes quite different effects? For example, some spells will have additional duration, some spells will become AoEs, and some spells will just be more spell. For example, invocation of nehek goes from an OK single target heal for undead units to a broke as heck AoE heal if you overcast it. Similarly, some lore of metal spells turn into AoEs. You probably know this, but it’s good to confirm.
Secondly, each lore of magic functions pretty differently, so looking for standard approaches is kinda wonky. For example, lore of life has a heal, same as lore of vampires. But earthblood doesn’t resurrect models, and invocation of nehek does, so earthblood is better at the start of a fight to keep units at top shape, and nehek can be used when things have gotten towards lategame.
You generally want to use magic to either mitigate a strength the enemy army has, or play up a strength your army has - bonus points if you can do both. By this I mean, don’t waste a buff on peasants. Don’t use it to turn rubbish parts of your army into mediocre bits of your army.
Don’t waste a massive damage spell like fate of bjuna or final transmutation on goblin wolf riders.
do, if you’re playing wood elves, spend an overcasted shield of thorns to give your shieldless, high damage wardancers +44% weapon damage and +44% missile resistance on the charge, because they’ll benefit more from it.
Do look for hints about what your enemies strengths are. For example, if you’re fighting empire greatswords, and you’re playing as the wood elves, don’t cast stone skin for +60 armour, because it won’t do anything. More armour doesn’t really help against armour piercing units no matter how much extra armour it is, because their attacks just ignore armour. However, if you are the empire greatswords, and you’ve got a fire sorcerer, then make sure you’re dropping flaming sword of rhuin. Wardancers and many many other wood elf units (dryads, eg) have a small (10-15%?) physical damage resist, and flaming sword of rhuin makes your units temporarily deal fire and magic damage (these are not the same things). Magic damage goes right through physical resist, so is the equivalent of just getting 10-15% free damage on top of the bonus magic damage anyway. And against dryads or anything else made of wood, well, fire damage should be fairly obvious. 
don’t always just cast spells. Holding off on aspect of the doomknight (lore of death, makes a targetted unit cause terror) till the late game where everyone’s morale is low is much better than casting it early on when everyone’s morale is high.) But keep an eye on what spells you’re bringing to the table. Lore of life has a generally low magic cost and benefits from being used early and often. Similarly, little waagh (from goblin shamans) is cheap and benefits from just being used all the time.
In short, look at what lores of magic you’re bringing to the field because even though lots of them have overlapping effects, as a collection they all work quite differently. Look for the moments of biggest effect and cast them. Buff your impactful units, not your expendable ones, and debuff enemy units that counter your impactful units. Work out whether the lore you’ve brought is cheap and spammy, or expensive and punchy. Don’t forget your overcasts.
Look at wherever you’re worried about on the battlefield and use it to turn those fights around.
