phaolo: --
Btw, I've read a small guide about the spells.
Can
Wind Walking make melee heroes reach dragons?
Nope. Sorry. It only lets your heroes float over terrain. Just bring archers/ballistas/your own flyers/use spells.
Maybe the air spheres would have been better then, as high level water spells seem a bit unimpressive (except for Geyser and Liquid Form).
These seem interesting too: Summon Great Eagle, Winds Of Fury, Shock Wave (dmg?), Call Hero (what level\cost?).
* The Great Eagle is a good scout, but useless in combat.
* Winds of Fury is fun to try and throw defenders over the city walls, but not nearly as good as the water spells.
* Shockwave is utterly useless.
* Never had much luck getting Call Hero to work. I usually end up getting some hostile hero I have to kill. Or it says something about "no hero available." Honestly I never bothered with it after the first few failed attempts, so perhaps there is a trick to it.
All in all I don't think air magic is much use, though Air Mastery and Air Elementals are pretty great if you have dragons or other flyers.
As for water magic: don't forget Great Hail and Beam of Frost. They're useful against massed enemies. Ice Shards also remains an excellent single-target spell. Can hurt dragons, with a bit of luck. Finally, Chain Lightning often is one of the best attack spells to use against regular foes even though it's level 1. Won't hit as hard, but the stun is invaluable.
My favourite water magic story, though, is casting the Flood spell on an map full of narrow islands and land bridges, and in one turn drowning every enemy who stood on the shore. Which was about half of them, must have been 50+ units total. Also drowned some of my own units. Fortunately, I was playing Lizardmen at the time. You need 4 spheres and a lot of luck for that one to work, though.
Also, what should I transfer to the next mission? Are the available units random?
I chose:
1 hero, 1 halberd (+1dmg, 3 strikes skills), 1 ring of movement, 1 fairy (flying).
Interesting choices would have been:
- a secondary hero (low skills, maybe too expensive to keep in the future).
- a crown of leadership (skill), an armor (+2def, +1a), shield (+1def, +1r), mace (+1def, 2 strike).
- a titan, avenger, valkyrie (weak), a wisp (neutral)
Ring of movement seems rather useless. Low level heroes aren't much good either. Easier to just find a new one in the next mission. The fairy will be helpful as a scout, though it's too weak to do much in combat.
I'd have brought the Titan, if at all possible. They're perhaps the strongest tier 3 melee unit in the game. Fast, round attack, first strike, amazing stats. Cast some buffs on them and they can take on armies single-handed. They're also immune to fire, so they can beat up Red Dragons.
The wisp is also a good choice. They're deceptively tough because they have so many immunities and resistances. Neutral isn't a problem: because it's not a normal Frostling unit, nothing you do will make it like you or hate you, so it will never desert. (This goes for all units with the "Creature" description. You can make the Undead like you even if you're neutral or good, by migrating cities to undead and building walls and such. But demons are Pure Evil Creatures, not Pure Evil Undead, so will never like you.)
The items usually aren't worth it. You can find new ones on the next map, or just rely on buffing spells.
As for what units get selected for transfer: I'm not sure how the system works. But what I do is simply disband every unit I don't need or don't want to bring right before I end the mission, so I'll be able to always bring my favourite gold medal units.
phaolo: Thanks, but I've just started and I'm already struggling in the first missions. XD 
