i play mostly fighting heavy armies, so i am used to rushing forward and punching things in the face. When it comes to magic i often get disheartened to see my troops die in combat, but get nervous chilling in the back. How do i use magic armies effectively?
If you like to punch people, consider using buffs, even a skink priest using Luckomancy can swing the tide in your favor, Portent of far and Second Sign of Amul help enormously in fights, maybe as much as a hero in some cases. Some of the other buffs the Slann can pull out are also beastly, Lore of light can cause WS 1, and Metal can give -1 to Armor save, Hit and wound. Getting those debuffs and buffs can let even skinks fight even on swordsmen, and saurus can be one upped over chaos Warriors. You can just Mass up on flaming pew pews and throw lightning and fire and comets at people, in this case, give all your skink priests at least one power stone, that way they can threaten to cast comet, or work two spells in one critical turn, typically right before a melee. What I like to do is go for spells that don't pew people to death, but spells that just say "I win" if they go off. The wolf hunts, Unseen Lurker, Pit of Shades, The beast cowers, or even Doom and Darkness can just say "you lose". A slann also lets you hate draft, facing a gun line? Take lore of life, spam rainlord on cannons and gunners and hide behind a unit that has the howler wind cast on it and abuse the fact that they'll be on a hill for master of stone. Ancient stegadons are nice too, but whether you use them or not is up to you, if you are all out magic then sure, use one to make heavens easier, maybe even have a slann take heavens and just predict a lot of stuff. Heavens can get stronger if you use the Skinks to cast portent on a Slann and use second sign of amul as well, with a great number of re-rolls powered up the lightning spells can really hurt, though the same is true for any damaging lore, Rule of burning Iron re-rolling ones? That'll hose someone for sure.
DonkeyHotep speaks the truth. If you are making a transition to more magic heavy lists I would highly recommend sitting down with the BRB and reading each of the spell lores very carefully. Start thinking how you might us each spell and you might start to generate some neat ideas to try out. As a rule I like to change up my lores regularly just to try new things and potentially discover some cool combinations I hadn't considered before. However, there are a few that just scream take me versus some opponents. Against a dwarven gun line Life sits high on my list, as does Metal for example. Another example, against Wood Elves, I like Life quite a bit for rain lord, howler wind, master of stone and the tree attack one (forgot is name). Another point to consider with magic is go big or go home. If you want to have a good magic phase take the appropriate choices and items to maximize that potential. Don't take a single Slaan and no priests and expect to make much of an impact. Also expect your first magic phase or two to be sluggish if you opponent is going heavy magic defense, but you can tease and force those dispel scrolls out of him and get on to working his/her army with your magic.
What kind of army are you planning on? How much are you willing to spend on magic. If I go heavy on magic, then I go all the way with two ancient stegadons, a focused rumination slann with 5 spells and maybe higher state of conciousness + a couple of enginepriests with some nasty items. Then I might imagine taking a few blocks of saurus warriors, some skirmishers and maybe terradons (and possibly kroxigor-buffed-skins). The idea is to stay home and throw a ligtning storm ontop of the enemy army. Skink priests draw away the enemy dispel with their ligtning and re-roll spells and when they are finished you make an evil grinn and unleash an infernal storm of destruction with the Slann. Use the first engine to grant protection for the saurus blocks and the second one to decrease the casting-values of the lore your Slann is using. If you want to max out the effects of the engine, take the lore of heavens for the slann. However i find that metal, fire, shadow, death and light are often better choices than heavens. Shadow is my favourite lore, as it is so verstile, it has many kinds of spells and all of them are useable and effective.
I used to take a slann and 3 scar vets, and just let the slann handle the magic phase back in the day. I used him as essentially a giant scroll caddy that carries the BSB and leads really well. I used to have him chunk a bunch of the first spells from each lore with +1, and not even get any gens. Now, the aura of power (+1 dice) is near mandatory, it's just so good, and the Cupped hands are also nearly mandatory, because soul of stone isn't gonna cut it. The slann can chunk a whole lot of Crap at the enemy by himself, and the old plan used to be go go scar vets, tank the enemy, nowadays I'm not taking that many, since skink chiefs and priests are a lot more useful. Further, you almost don't need more fightiness since los Hombres Lagartos y los eslizónes pequeños can kick a lot of ass on their own, they don't really NEED fight heroes against most armies. I also recommend thinking about tempo, most enemies have a limited number of dispel scrolls, and once their gone, a lot of what you have will just get through, you can also make people hoard dispel dice by saving a powerful spell for the end of the phase.