Mika, my favorite mages are the priestress and the warlock. They are also my most played mages - we could play a match via OCTGN to test your claims.
I play priestress you play warlock. Because in my opinion the priestress is the strongest mage and has a good chance to win in every setup, even against necromancer and warlocks.

I really like the idea of the "skirmish" trait. Attacking and then being able to move it a nice, strategic option but not too strong and far away from being imbalanced.

There could be some creatures with that trait too, in the future. Who knows.

Because you can have more attacks in one attack-action (e.g. battle fury) I think only the first attack misses.

I think the Ballista is a bit too strong too.

Normally you would have to pay 7-8 mana for an attackspell that has 5 dice and 3 piercing.

And if you time it a bit, you can use the ballista nearly as good as an attack spell. Final quickcast phase you could play it and use it right in the beginning of your next Action phase.
The problem is, the enemy has so respond immediately to this threat, somehow. Because otherway you get a 7-8 mana attackspell for free in 2 rounds.

I think starting with 0 load tokens would really decrease the pressure these ballistas can generate. Because one round of preparation really makes a difference.

And I also liked the idea of a Goblin worker who can charge the Ballista faster. If there are 2 goblin workers and they both spend their full action charging the Ballista, then I am fine with it shooting every round. It is kind of exactly what these small Goblins are good for. Nocking arrows in a warmachine and maybe even taking damage by doing it ^^

Because the Ballista is still a promo I hope it is okay resurrecting this thread.

There is a mage guarding and a deathfang guarding in a zone.
A Gremlin wants to attack the mage, can he do it?
Or can the enemy mage decide, that the deathfang has to be targeted?