Others like Fimbul winter has little strategy involved except for one thing. no towers or TCs or fortresses can fire arrows while it is cast. So don't waste this GP as soon as you hit Mythic age, save it for when you assault their town to prevent those fortresses and TCs from firing. the wolves will be excellent distraction for enemy soldiers and give your rams enough time to get up to the TC and destroy it before it unfreezes.
Meteor. I have seen so many newbies use this one wrongly. first of all. don't cast it as soon as you hit mythic age for that little boost in the battle. it doesn't hurt units very much. also, don't don't cast it on your enemy town unless you plan on following up with an attack army and siege. Meteor is good at crippling houses and farms when they are built clumped together, but if you don't hurry and take advantage, then they will be replaced.
Lightning bolt. don't waste this in the second age when the enemy fields a cyclops. your weak little units are worth the price of dieing to save that bolt for a more powerful unit. just make a hero and kill the cyclops. no need to waste bolt there. save it for nidhogg or son of osiris or some other hard to kill unit. Its usualy not a good idea to cast lightning bolt on the pharoah because he returns, but it can help if hes killing your myth unit in battle and he has no other priests.
Ragnarok is the big one. should you use it at all? when? First of all, don't cast it until you have enough food and gold to rebuild your villager and dwarf economy after the population has died down. and try to make as many vills and dwarfs from all your TCs so you get more benefit. and pump lots of cavalry and portable rams because you won't get to make these after ragnarok is cast. make sure your population is full to get maximum effect. if you have no military and 80 villagers/dwarfs thenyou won't be as strong as if you have 10 portable rams and 80 vills.
Restoration is often used in the beginning of the game during a rus to keep their cheap little cavs alive for an extra 30 seconds. ugh. big waste.
Save it for mythic and the big battle when hundreds of units will benefit fro it. same for bronze and flaming weapons. don't use them during small skirmishes. save those for the big battle.
locust swarms. This seems to be one with no side effects, so its good to cast anytime. Cast it early, but only when theres lots of farms clumped together.
Loki's spy. its best to cast it on a strong unit like the pharoah or Ra priests that will be sitting in the heart of their economy for a while, or a norse scouting ulfsark, good to see where he builds the forward bases.