The Amulet of Dark Whispers is a near must-have for mages. It's the only way for mages to get stun immunity. And a stunned mage, is quickly a dead mage.
As for your advanced skills, based on what you said you want, Earth Mastery and Summoner are obvious choices.
Body Development can help make your mage a little more robust. It requires END 4. Body Development also pairs well with Massive Critical (Massive critical requires STR 3).
For mages, I think Precision Shots is more useful than Massive Critical, but that's a personal choice. I often invest 1 skill point in each at first, than put 2 into Precision shots. Late game, might increase massive critical.
1 skill point in Death Cloud can be useful as a defensive tactic, plus extra damage when using death weapons. Since it lowers a monster's resistance to all death attacks, it can even help your companion if he or she is using a death weapon, too.
1 skill point in Mage Barrier can be useful as a defensive tactic, the push back will help keep you from getting trapped by a mob. Investing more points gives you more wiggle room and some extra damage for attackers.