Physical durability for mages

I would still get seal. It’s honestly an amazing skill.

Besides, not all enemies can be frozen.

I’ve played it a lot and my experience is that its only good against stopping small damage numbers, so basically from big groups swarming around you. But I never had troubles with small groups because of Arcanist’s + Inquisitor’s AoE potential. When I max out the Seal it stops 280 damage per hit, which is almost nothing at level 85+, especially from heroes and bosses.
Basically said, I don’t find it remotely worth spending 12 skill points on it. Especially if it would cost me my skill points on Trozan’s Sky Shards, which for me deal 40k damage AoE each, which is quite a lot for a 2s cooldown cheap spell that also freezes enemies.

You should really max Deadly Aim and one of the Exclusive Skills though, it’ll make your build better in the long run. In your case, either Star Pact or Aura of Censure should be good.