Optimal builds for fragile companions?

I'm very late to the party, and suspect the OP has long-since moved on, but for the record Leliana works far better as an archer/Ranger in my playthroughs.

In the early to mid-game I focus almost exclusively on her utility, concentrating heavily on Device Mastery and Stealth, and pumping most stat points into Cunning and then Dexterity, so she'll only have basic archery skills for quite a while, and her DPS will be lacklustre. But that isn't her role. Her role is to scout ahead, spot potential threats, disable traps and help level the battlefield in the party's favour. In combat she keeps well to the back and uses a simple shot rotation that I set up on the tactics screen. The Ranger pet helps immensely as it's like having a fifth party member to help out. I'll rarely go any higher than 10 points in Constitution, but I do like to get a few more points into Stamina over the long haul as those Archer abilities use a lot of energy. I also usually give the Battledress of the Provocateur right off the bat.

Late game I begin putting more points into actual damage, rounding off her Archery abilities, putting some points into Bard for the party buffs and by the end she's quite a powerhouse. It just takes a while to get there.

Zev, by contrast is a pure damage dealer, and I'm going to be pumping up his Cunning, followed by Agility and focussing on anything that boosts two-handed daggers.

Sten is a funny one. I don't use him much, but initially I like to have him as an archer, but this time focussing exclusively on damage-dealing. It's only after getting the Effort armor set from the Deeproads and get his sword back that I'll respec him into a more traditional two-handed fighter. This is because I kind of like the idea of him not feeling worthy enough to wield a sword until his own is returned to him.

Now Morrigan.
The key to DA:O is not pumping out as much damage as possible. The key resides in tactics, careful positioning, and having the right tools for the job, and in my opinion one tool you're absolutely going to need is anti-magic.

Therefore Morrigan, for me, specialises in anti-magic. Removing enemy mages from the battle, nullifying them, or killing them quickly. Her other role is of course crowd control, and finally she can damage and she can heal. She has a lot of roles in the party and she needs to be competent at every one. But enemy mages can absolutely wreck your party at higher difficulties, so to my mind being able to deal with them is paramount.

Obviously I use the respec mod, and would never dream of playing without it. It's the one mod that's absolutely essential to me as Bioware's default builds are not optimised at all - in fact they're all over the place.

My first goal with Morrigan is getting her armed with some basic crowd controls and at least a basic healing spell. For stats I'll typically put three points into Magic for every one point I put into Willpower. In fact levelling her is often a simple process of two Magic/one Willpower, though I'm also going to want her to have around around 12 Constitution at some point.

Forget about Shapeshifting, it's next-to useless. I just get rid of it, and give her Spirit Healing instead and she'll get Group Heal as soon as it's available.

I'm going to want Paralyze right away, along with Mind Blast. She's also going to need Telekinetic Weapons, and above all, Forcefield which is the one spell that is 100% mandatory for all Mages. I'll also need to get Glyph of Paralysis pretty soon.

For damage she'll start with Winter's Grasp which is a great spammable default spell, and I'll get her basic Lightning Bolt and bog standard arcane bolt. I'm then going to be working toward getting her Crushing Prison as soon as humanly possible, followed by the awesome, overpowered Mana Clash -- the bane of enemy Mages.

Once that's achieved I can relax a bit and just give her whatever I think is cool. I'll generally work toward giving her more Primal abilities from the Lightning and Cold tree, until I can use her AOE spells effectively. Walking Bomb is also great fun. At higher levels Glyph of Repulsion, coupled with Storm of the Century is wonderful in clearing out entire rooms of enemies. Leliana scouts ahead in stealth, exposes the enemies, shuts the door... then Morrigan runs to the entrance puts down the glyph in the door frame, and then summons the Storm. Hilarity ensues!