Metal is pretty situational, against heavy cavalry or dwarves, even empire, sure it's good. The problem is many of the spells can be completely worthless, even the one that destroys magic items can be useless. However, nothing changes a game like getting 11 wounds on a spirit of the forge against chaos knights.
Beasts is a lore that I think is kinda funny, it makes stegadons and saurus cavalry terrifying with the last spell, and it's on the cheap side. Even a cracked up skink chief can whoop face with bears anger, and usually there is SOMETHING to make cower, or take a stand, but it's pews are not particularly great.
Heavens isn't that bad a lore, all things considered. It's hard to cast, but a slann should be throwing tons of dice anyway. This is a great lore to know all 6 spells of, since they are evenly split between offense and defense, (luckamancy anyway) you can just change the roles to conserve dice. The infinite range of the pews is nice also, and let's you be a jerk with the slann placement.
I like fire, nearly every spell in the lore is good for something, especially if the something you want involves str 4 flaming hits. Elves of every type need to be given a huge dose of this. Trolls, Treemen, Mummies, Lord Kroak, That one dark elf character with the mask of sea gold, you never know when fire solves your problems extra well, but it should be noted that the conflagration has infinite range, and arguably no targeting restrictions. Unfortunately, it has 2 RIP spells, so don't wall anyone if toad is gonna whoop out his lightsaber (though fire toad can actually win quite a few duels).
Light, I personally don't like this lore for a toad, mostly because that hammer fist spell is TOTAL CRAP, and it's healing spell is only occasionally useful. The 6th level spell is similar to the burning alignment, but more and less armor ignoring, and the first level spell is pure gold against it's respective str 6 targets.
Death, I've had good luck with this against the French, ignoring a 2+ armor save is quite helpful on it's ultimate, it has a lot of pews, and masters people with bad armor.
Shadow. I have no Idea what's up with this spell list, but I generally don't take it. It's final spell is completely cheap and can win the odd game by sinking a vampire count on a 6, though it's primary use seems to involve killing other toads
Dwarves are a fair choice also. Shadow can make people go places and it has a really questionable area spell with crown of taidron, which in my opinion is TERRIBLE for a slann.
Life..... Unless your Slann is modeled with a fairy gown and a tall princess hat then leave this to the fair maidens of the French armies. It's almost always crappy and I hate it, there are few things I can say about this lore that aren't bad, but here's a few of them, which are as painful for me to type as a 40 foot line of shipping chain would be to pass through my bowels. The 1st spell is helpful for making people go nowhere, and is effective even against aquatic models, for some reason. The rain lord makes empire sad, if for some reason you feel that making half a cannon's shots useless. Wind wall is by far the best spell if your army is composed of lines of knights, and your enemy has guns. This whole lore seems to be based around making gun lines shut up and sit down, and it deserves some respect. If your Bretonians.
try getting the saurus unit with Portent of Far from a handy skink Priest and get those 1's re rolling and watch them mince elite infantry and even Heavy Cav, just through weight of wounding hits! Not as hard as you'd think to get set up either with the opponent worrying over Cleansing Flare...
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