Okay, I am up to level 8 now and I've just defeated the Turtle using Fire Arrow 2 (can't use Fireball 2 on it which is annoying), as well as Inquisitors, Priests and Thorn Hunters (plus Royal Snakes and Swordsmen which mostly defended until the final round or so).
Things are going quite smoothly. I suppose a Mage on Impossible is going to have the lowest possible leadership in relation to the size of the enemy stacks, which means that most enemies in Darion are listed as Match, Slightly Stronger, Strong or Very Strong. And these are just the normal enemies that you would easily mop up on lower difficulties. Despite this, I have virtually managed to clear out Darion now, except for the usual things like Rezo's Tower, Abandoned Castle and Magic Range. I have a few more things to tidy up but I will certainly be level 9 when I go to Freedom Islands. I am a bit of an OCD player and I don't like loose ends, i.e. enemies wandering around.
I had wanted to have worked on Trophies and Trading by now, but a lack of Mind runes is stopped me from doing this. I don't know why, but Might runes seem much more common than Mind. I'm not sure if this is random or not.

I think I will only put one point into Trophies, but I will definitely max Trading. I know some people don't think it's necessary but I hate selling items at less than full value. I tend to hand onto them until I have Trading 3 (not scrolls - you pretty much have to keep selling them or you can't buy anymore).
More importantly than the Mind tree is the Magic, of course. I have Chaos 2 (doesn't need Mind runes) and Distortion and Order 1. I am using Fireball 2 a lot at this early stage. In fact I think I may go straight to Chaos 3 as soon as I get more runes. I've got Kamikaze but I need Phantom in order to use the 'terrorist' trick that has been discussed in another thread.
I have had VERY good luck with Feanora's children. In past games I always seemed to get 'scroll children' from her, but this time I have 3 and they are:
+5 defense
+ 20% defense
+ 20% intellect
This is awesome!

So I have 4 Attack, 14 Defense and 13 Intellect already.
I also got the belt which gives increase in morale to Priests and Inquisitors, and even better, something called the Banner of True Faith which was expensive at 60,000 gold but it gives I think +4 mana, +800 leadership and 10 Magic runes after 50 victories (48 to go, heh). So I have 3500 leadership already, which is awesome for a Mage this early. And if I can get to Chaos 3 soon, with about 70 mana I think I am ready to dominate much larger enemies.
Bring it on.
EDIT: the fourth child gave +20% mana, which is unbelievable. Feanora's children are so good this time I may have to keep them for the whole game!
Even better, when I got to Freedom Islands the king dude there had Ice Snake, so now I'm dealing 1430 damage plus 400 or so to adjacent squares. (I have got Chaos 3 now and 15 intellect). This is at level 9, remember. So I am sailing around the Western Islands picking up stuff and smashing about half of what I can see straight away. This is a domination...on Impossible
