I have a question about race creation, Can I be Krax blood and still get 'life' magic?

Yes, you can create a custom Kingdom faction (life magic) with Krax blood.  Note that not all bloods can "switch hit", but Krax is one that can.

Some picks that help a damage spellcaster sovereign are:

• Brilliant - mostly for the level bonus to spell mastery, though the experience bonus doesn't hurt
• Attunement - +2 mana per turn can help a lot in the early game
• Tactician - +1 initiative never hurts when trying to get your spell off first
• Warlock - +25% spell damage
• Procipinee's crown - can carry lots of enchantments (and summons) with no residual cost 

With regards to race for a custom faction, for a damage spellcaster pretty much any race is fine except Ironeer (with their extra mana costs) and even there it's only a handicap, not a deal breaker.  You probably want to determine what your "big gun" is going to be; if it's an elemental spell like Blizzard, then there's not much you can do to improve that with the base game's factions.  If you're going for death magic, choosing death worship on an Empire faction lets you corrupt shards to improve the effect of several death spells (though horrific wail is perhaps the most efficient and has no death shard factor - just level.)

I would certainly not pick 4 schools of magic.  I'd pick 2 at most.  You won't have enough XP in a typical game to "fill out" more than 1 or 2 schools of magic (although if you can buy level 1 from Procipinee's crew, it can be nice to have "free" level 1 spells in all the elemental schools.)

I'd recommend something like Water1+Life1 or Air1+Death1 on your sov.  The water and death give good army killer spells, the air and life give good support spells.  

In each case, you want to beeline to your army killer spell (Blizzard for water, Horrific Wail for death/necro, Fireball for fire, etc.) Then focus on casting time reduction. Then focus on damage multipliers, spell cost reduction, better spell mastery, improved initiative (from general), and rounding out support spells.  After that, you can choose what looks useful/fun

Mages can be great dragon killers, but you might want some fodder to slow the dragon down while you prepare your spells.  Just don't expect to fireball it to death  They can also act as force multiplier to let your other units work more effectively - fire resistance buffs, debuffs like slow/curse/blindness on the dragon, buffs like haste/stoneskin on your units, summoning cannon fodder to slow the dragon down - these can make a big difference.

Some good advice above.

Your Race does not influence your Mage potential much. Choosing Kingdom or Empire is much more important: Kingdom has Life Magic, with healing and a great city enchantment. Additionally, a Mage can learn to summon a healing unit and a powerful defensive unit.

Empires have Death magic, with lots of debuffs and a Dragon killer spell at level 5. It does not have mass damage spells though. Evil Summoners can summon lots of skeletons and learn a mass damage spell, Horrific Wail.

Your Sovereign and Faction Traits are also important. Brilliant is an obvious pick, Attunement and/or Procipinee's Crown are very handy. For your Faction, anything that gives additional spells, like Death Worship or the Decalon is nice, but not a necessity. Enchanters increases your mana income and is probably the best trait out there, even if you are not a mage. Flesh Bound Tome can increase your mana income as well, but it's a bit OP for me. Lastly, I have a fondness for Wanderlust, as quests can give nice items and help you gain levels, which you will need as a Mage.

Personally, I always pick Summoner instead of Warlock, as the summons help me survive the early game. It's also a lot less mana-intensive. But I try to play at a very high pace, fighting as many battles with my Sovereign as I can.

There's a thread on this forum about favorite Spell Schools. I don't have the link, but it shouldn't be too hard to find. It's a fun read and should give you some ideas about the pros andcons of each School.

I like the Air+Death combo wizard and will use it as support with lots of buff/debuffs. Typically I use my heroes as support and leave the dirty work to my regular troops. As such I like Defender heroes and support wizard as sovereign. Seems to work well in this game. 

A Defender hero + guard ability is insanely strong with good use of Guard skill. Can turn a row of militia into a sturdy wall.

Agree! Summoner mages are powerful and a fun alternative way to play a mage. It's also fun to summon powerful units that are also expendable, so you can be more aggressive with them. Summon a unit in the middle of their archers. Even if it dies, you've distracted them and kept your regular units safe from their arrows for a few turns.

I also recommend Procipinee's (sp?) Crown so you can strategically summon a unit and not be saddled with maintenance cost.

However....if those summons stick around for several levels....especially the Air Elemental and the Earth Elemental.....they become very powerful.

I tend to also like throwing Cloak of Thornes and Fire Mantle on the summons so that they deal damage on the defense aswell.

To the OP: I've been doing this for Mage and it does works wonders:

Play a Life-Magic Mage, make him/her a Summoner. Choose trait "Summoner+" so you start with a bit buff for summons. You'll want to get the Attunement and Brilliant traits along with the Summoner profession. This is to help on leveling, which is important to get the advanced spells earlier. Don't focus on spell damage as you won't be doing any damage. Also get a staff with ranged attack. For cloth, it's your choice. I pick up the usual mage robes and put my Archmage on a horse

Focus on Heal ONLY, grab as many Life Shards as possible. (Don't choose any of the Evoker/Spell Mastery crap, just Summons). At the very first level, choose the 25% XP trait (Knowledge) Believe me. You should have a Shadow Warg and few units from starting game to clear first round of creeps and should level each battle. (provided your sovereign is the only one on the party - no champs).

When the opportunity comes early in game, you will have the following main summons: "Wisp", "Air Elemental" and the "Lightbringer". These are going to be your mains. Grab a few Defenders to add to Damage or Archer unit. DO NOT add any more champions on your party. Give those their own army. EDIT: Please cast your summons on STRATEGIC so you'll stick to them for the rest of the game and won't use much mana. 

Your battles should be like this: Air Elemental tanks along with other units and Lightbringer, who will cast Blinding light to blind enemy. Wisp backs up your sovereign heals with timed healings when appropriate. Wisps' healing is AOE 1 tile. If you play smart, your units won't die due to constant healing. Something to say about Air Elem. is his levels add to his HP up to 100+ and also he's specially deadly against Demons who suffer from Lightning Damage. With "Growth" you can kill demons in two hits.

NOTE: "Healing" spell costs only 9 mana.

Further in Life Magic, when Growth and Shrink becomes available, cast "Growth" on your Air Elemental, "Shrink" on the most powerful enemy on battle. By mid-game, your Healing Mage should have/use the following spells smartly:

Regeneration, Growth, Shrink, Healing Spring, Heal, + Summons: (Air Elem., Wisp, LightB, Grave Elem., etc.).

I've been using Tarth for the "Ram's Bow" because I throw archers in my party. My army isn't Deadly or Epic, but I can kill pretty much anything but dragons. I've managed to participate in the "tournaments" that you can find across the game. (They're "Epic"). Until mid-late game, you're strong enough to attack/defend anything.

I hope this helps you. I can put screenshots if needed as this is the setup I use every time I am not using my Assassin build. 

1.) Pick mage

2.) Win

But seriously, ok...

1.) Pick mage

2.) Get Aeromancy

3.) Get a champion to water3

4.) Cast Mantle of Oceans on your mage

5.) Cast lightning bolt over and over, win every battle

Pretty much this.  Although if you build a decent number of conclaves with a lot of essences and build apothecaries, you don't really need Mantle of Oceans.  So, its really only 1. Pick mage, 2. Get Aeromancy, 3.  Win.  The 20% damage increase perks make lightning bolt deadly.

Combine one mage with lightning bolt and another with summoning (for distractions), and give them both daggers that boost their initiative, and you can win easily.