Regular FFH doesn't use great generals like BTS does - instead, you can get some GG great people points from certain wonders. Edit: Fall Further uses the BTS GG system (the great generals have been massively modified with the latest release though), and orbis also does so. Look at form of the titan (world wonder) in FFH and you will see one place you can get GG points from.
Now, to protecting your mages - as said above, you need either A. stoneskin, or B. a unit with the guardsman promotion. The guardsman promotion requires combat 3 or 4 as a prereq if I remember correctly - the best way to get it is to put it on a hero or to get a unit that starts out with it. The only unit that starts out from it (iirc) is the Royal Guard (available at feudalism and only with the aristocracy civic). However, you SHOULD have gilden by now - put combat on until the guardsman promotion is available, then escort all your mages with him. This will make him useful even in the late game because all guardsman does is make the highest rated defender the one used - so if gilden was guarding some mages, but there was a combat 5 phalinx in the stack, the phalinx would be the defender (needless to say, I doubt the assassin would kill that

).
Now, on to attacking cities. The Ljosfalters don't have seige equipment, which means that they have to use magic to buff their troops and seige with. There are a few spells that you should keep in mind when doing this:
Maelstrom (Air 2) - you've used it, it damages all enemies in the area.
Fireball (Fire 2) - it causes collateral damage, OR you can use it to bombard city defenses.
Blur (Shadow 1) - makes your units immune to first strikes and defensive strikes.
Shadowwalk (Shadow 2) - you units ignore building defenses - this effectively makes them gunpowder units, you will still have to worry about cultural defenses and terrain defenses, but you won't have to worry about walls palisades, etc.
Specters (death 2) - strength 3 unit with +1 death mana affinity, can be spammed as cannon fodder
Host or Erehnjar (sp? - Law 2 summon) - strength 4 +1 holy summon, again usefull as cannon fodder
Rust (Chaos 1? or entropy 1 - don't remember) - this spell removes metal promotions from units on the stack selected - very useful debuff
Enchanted blade (Enchant 1) - +20% strength to melee units
Fire Arrows (Enchant 2) - +1 fire combat strength to archery units
Dispell (Metamagic 2) - removes all magical buffs on units within 1 tile
Poisoned Blade (Nature 2) - +1 poison strength to all recon units
Summon Tiger (FoL priest spell) - summons a strength 4 tiger (permanent summons, will recieve a +10% strength bonus per level of combat from the summoning priest, plus can recieve xp and promotions of its own - useful as cannon fodder.
Those are most of the spells that you will find useful in attacking cities. Generally when I attack cities I bring lots of mages with fireball spell, as that is the easiest to use and I can spam fireball until their defenses are down and then spam more to cause collateral damage. However, you can work it with air and shadow, with significantly fewer mages, as you need 1 adept to cast blur, 1 mage to cast shadowstep, and then a few mages to cast maelstrom.
Personally, when playing the Ljosfaltar, I like to play with mostly archery units. This means that when I attack, I have to do it with their archery units. Generally this means that I have to either overwhelm with archers, or I need mages. If I have mages, I will have at least 1 of the above mentioned methods - and if I rush mages, get enchantment mana and fire mana or shadow mana (Palace provides air, nature, and I don't remember what else), Enchantment 2 means that my archer attackers are now attack 5 defense 6, and some sort of collateral damage means that I can penetrate the defense. Otherwise I usually wait till I have longbows, and longbows with fire arrows are strength 7 attack, 7 defense, + 1-2 for metal. This, combined with maelstrom or fireball, is generally more than enough to penetrate most defenses for a while. There are definitely times though when it isn't optimal to attack - generally the best time is right after you got a new tech (like bowyers or somesuch), and have upgraded your force to be best usable by them.
Hope this helps.
-Colin