Best build for fast combat?

Blood mage/arcane warrior, in that order.

Early on, go heavy into electrical and cold and push for mana clash. That will get you 'Storm of the Century' which is the single best aoe spell combo there is. If you push Morrigan down the cold branch and Wynne down the electrical branch, you should be able to cast it repeatedly by switching off casting partners. Have Morrigan grab all the hexes and Wynne grab all the glyphs. After you fill out your Storm o' the Century requirements, concentrate on the line ending with Crushing Prison. Crushing Prison and Mana Clash will take out the vast majority of casters. After that, the fire line can be fun, I'm unduly fond of Grease. What you should avoid is healing. Wynne takes healing. Neither you nor Morrigan need it. As satisfying as it is to heal a companion, any slot used for a healing spell is a slot wasted by not debuffing or doing damage.

While you are filling out blood mage, use Shale or Alistair, Leliana and either Morrigan or Wynne for companions. After you ascend to arcane warriorhood and can wear armour, ditch Shale/Alistair and use Leli, Morri and Wynne. When you are looking at armour stats, remember that stamina = mana. I use Leli because I have a compulsive need to open all the things. If you do not share in my OCD, feel free to substitute a damage dealer such as Zevran, Oghren, ArfHowl or the Sten.

Hope this helps!

Well, I'm not really a combat expert (especially not in Origins) but if you want to go fast I'd suggest getting the Momentum talent from the Dual Weapon tree. Also use daggers.

I am not an expert on which bugs were in the xbox release, but if it was like the PC version, you can end up hitting the attack speed cap and getting booted back to "normal" speed if you try to combine Momentum with Haste. On PC we have the luxury of 3rd party bug fixes though.

But otherwise Haste is the single biggest ability that can speed up the entire game since you boost DPS and you even boost the speed at which you travel around between things out of combat.

I prefer the fire line for elemental damage on mages, frost predominantly for control with Cone of Cold (bug physical resistance check is not "rolled," which is why it works on nearly everything), and then I tend to skip the electrical line. Walking Bomb and Virulent Walking Bomb are my other main vanilla mage damage spells, especially because DOT interaction (at least on PC) can lead to a lot of extra bonus damage. Hexes to improve damage of whatever element.

I don't play an actual blood mage much (although always take the spec), but Blood Wound may be the single best mage spell. Mana Clash is great for magical enemies, but somewhat expensive and more of a mid-late game luxury for the player mage.

Paralysis Explosion (Glyph of Paralysis + Glyph of Repulsion) is my favorite combo out of any since provides CC to practically everything and the individual spells are good on their own as well. Sleep is ok CC, which is pretty cheap on Morrigan given how she is likely auto-leveled when you pick her up.

Although strictly speaking a warrior tank isn't necessary, probably simplest composition to micro has one + 3 ranged. The standard way to build would be SnS and many times recommended to be Dex pumped if you are starting out, although the damage is not very good and that tank should be running dagger to maximize the DPS. "Pure" strength SnS (min dex for shield talents) will also work as a tank, but will do more damage with shield attacks (using best of whatever axe or sword you have) and has the advantage of likely being able to equip heavier armor earlier in the game. Or you can even go 50-50 Str - Dex for fine all around character that has good defense and ok damage. Really it doesn't matter a great deal as long as attributes are going into either strength or dexterity as constitution is not as good an investment.

It is possible to build dex based dw tanks or off-tanks with either warriors or rogues, but probably doesn't make as much sense to do this if you don't have access to the ability to respec companions and redo their early leveling and attribute investment as you can with mods on PC. But that said late game Zevran or DW Warden can be decent off-tanks with relatively normal build progressions and no respecs. Warrior is still better than rogue as a tank because of taunts.

Also worth noting Force Field from a mage can make your tank immune to damage (including friendly fire) for a while, and is a way you can clump enemies together and then nuke them without killing your tank. If you don't mind abusing this, it also comes in handy against dragons as you can force field a character that gets grabbed by the dragon to keep them from getting chomped too badly.