1. The best way to make a battle mage without worrying about getting killed is to start as a (pure) man at arms then just breach out to fire and earth.
2.Depends on what difficulty you are playing (also if you are a Lone Wolf or not) , casters get that staring ranged attack that does damage based on the equipped staff (aka you don't care for the extra Ap cost from level diff.)Glass Cannon helps a lot and transforming terrain before the battle is great.
3.You should try to max out your primary stat (mainly for gear req.)but in late game you will most likely get at least +10 bonuses from gear , know that elemental resistances are important as well since you can always get healed by them without spending AP.
It is possible to play without melee character? How can i do it? I know that summon will help because NPC like summons but if will have ranger...
I think so but I don't recommend it ; If you do then get Charm and lots of magic scrolls (mainly for boss fights).
I still do not understand tenebrium. So, after some point i will be finding tenebrium weapons only? An they will be better than my crafted weapon?
So i do not need to add points to my weapons skills?
You will always find normal weapons in the game, tenebrium is just like any other element in the game witch means that you can enchant your gear with the tenebrium stat (be it damage or ress.).
The only diff is that tenebrium weapons have their own "class" so if you use a 1h tenebriunm sword you will not get the extra benefit from the single handed wpns you will get it from the one called tenebrium weapons instead.