Spirit Healer + Blood Mage is the best option for a pure spellcaster, though the two together make no sense from a roleplaying perspective. Shapeshifter is lousy for mages. If you want to be a melee mage, Arcane Warrior is the much better option. Shapeshifter doesn't even give you good stat bonuses for a mage (+2 constitution, +1 armor).
If you want to be a true Arcane Warrior, then take some time to really learn the spells. The Dragon Age wiki [http://dragonage.wikia.com] is great for that. You make heavy use of buffs, so you need to be sure you're using your mana pool wisely.
As a mage, I prefer to go with the pure spellcaster route. If you want to be more of a healing or party buffing mage, concentrate more on Willpower (that stat determines your mana pool). If you're more for being an offensive mage, go more heavy on the Magic stat (increases spell potency and effectiveness). When I say focus on one over the other, I don't mean neglect one in favor of the other. Build up both stats enough so that you can have access to what spells you need, and then put 2 points in your favored one and 1 in the other as you level.
Some must have spells:
-Mana clash: It's an area of effect spell that basically kills enemy mages instantly
-Heal: I give this even to my non healers. It's a tier 1 spell and always comes in handy
-The Arcane Tree: The first tier gives you a decent attack spell for early in the game. The second spell gives you a self-shielding power. The third tier increases your staff damage. The fourth tier increases your spellpower. All of these are useful.
-cone of cold: Will freeze enemies. Frozen enemies will shatter to critical hits or to the Stonefist spell.
-Stonefist: See above
-The Telekinesis Tree: Not a must have, but every spell is useful in the tree. Tier 1 is Mind Blast, which stuns all enemies around you. Tier 2 is Force Field, which you can use to protect a party member or to stun an enemy (who won't take damage while stunned) for time while you focus on other targets. Tier 3 is Telekinetic Weapons, which gives a bonus to armor penetration to your party members. Tier 4 is crushing prison, which stuns an enemy who takes damage while stunned in addition to any other damage your party throws at him.
-curse of Mortality: target takes damage over time and cannot heal.
-Death Hex: The whole Hex Tree is useful, but Death Hex makes every normal hit a critical hit. If you combine it with Death cloud (the 4th tier spell in the same tree as curse of Mortality), then there's a pretty strong spell combo. Just be careful of friendly fire.
There are other excellent spells as well, but those are some of my favorites. You'll want to spend points on the spells provided by your specialization as well.
Here's the list of spell combinations [http://dragonage.wikia.com/wiki/Spell_Combinations]. They're pretty fun to pull off, and are all very effective.
