Okay. Well here's what I think I would set them to in your shoes. I'm not saying they're optimal, (they really don't need to be in Awakenings) but they should be serviceable enough to do a decent job without babysitting:
Please feel free to tinker around, obvs. I can't remember offhand how many slots you get for tactics, so I may have gone over with one or two of them.
Also, I assume by 'enhancement' tree, you mean the one that ends with Haste...
Nathaniel
Behaviour: Ranged
Self: Health < 10% (make it 25% if no healer mage in party) - Use least powerful health poultice
Self: Stamina < 10% - Use least powerful Stamina potion
Self: Being attacked by ranged or melee - Decoy
Self: Surrounded by at least 2 enemies - Pandemonium
Self: Any - Activate Accuracy (if you don't already have this, get it ASAP. It's great. Aim until then)
Enemy: Any - Summon Spider (or Bear, or Wolf, if you prefer them)
Enemy: Target of Party Member (select warden or a melee companion) - Mark of Death
Enemy: Highest Health - Arrow of Slaying
Enemy: Highest Health - Critical Shot
Enemy: Rank Boss or higher - Shattering Shot
Enemy: Nearest visible mage - Pinning Shot
Enemy: Nearest visible (Any) - Pinning Shot
Enemy: Target of Warden - Attack
Justice
Behaviour: Default
Self: Health < 10% (< 25% if no healer in party) - Use least powerful health poultice
Self: Stamina < 10% - Use least powerful stamina potion
Enemy: Nearest visible mage - Holy Smite
Self: Any - Activate Beyond the Veil
Self: Any - Activate Shield Wall
Ally: Health < 50% - Fortifying Presence
Ally: Being attacked by ranged or melee - Taunt
Ally: Being attacked by ranged or melee - Guardian's Shield
Enemy: Rank critter - Assault
Enemy: Rank critter - Overpower
Enemy: Rank critter - Shield Pummel
Enemy: Rank critter - Shield Bash
Velanna
This one is a bit more interesting. She can be built to be an all-out healer, or more DPS focussed. I'll do two builds, but you can mix and match for something more in-between if you prefer. As a Healer, no-one in your party will ever die (like, ever), though that is rarely a big concern in Awakenings. You might well find that you're sacrificing DPS needlessly. But it's your call, obviously.
HEALER
Self: Health < 25% - Use least powerful health poultice
Self: Mana < 10% - Use least powerful mana potion
Ally: Health < 90% - Regeneration
Ally: Health < 90% - Heroic Defense
Ally: Health < 75% - Heal
Ally: Health < 50% - Group Heal
Ally: Health < 25% - Lifeward
Ally: Being attacked by ranged attack - Heroic Aura
Ally: Being attacked by ranged or meleee attack - Haste
Ally: Being attacked by ranged or melee attack - Heroic Offense
Self: Any - Activate Rock Armour
Self: Any - Activate Frost Weapons
DPS
Self: Health < 25% - Use least powerful health poultice
Self: Mana < 10% - Use least powerful mana potion
Enemy: within short range - skip to tactic 11
Self: Any - Deactivate One With Nature
Self: Any - Activate Frost Weapons
Enemy: Status Frozen - Stonefist
Enemy: Status Petrified - Stonefist
Enemy: Nearest visible - Winter's Grasp
Enemy: Nearest visible - Petrify
Enemy: Target of warden - attack
Self: Any - Activate One With Nature
Self: Any - Activate Nature's Vengeance
Self: Any - Activate Thornblades
Mixture
I'd suggest something like this:
Self: Health < 25% - Use least powerful health poultice
Self: Mana < 10% - Use least powerful mana potion
Ally: Health < 75% - Heal
Ally: Health < 50% - Group Heal
Ally: Health < 25% - Lifeward
Ally: Being attacked by ranged or meleee attack - Haste
Self: Any - Activate Rock Armour
Self: Any - Activate Frost Weapons
Enemy: Status Frozen - Stonefist
Enemy: Status Petrified - Stonefist
Enemy: Nearest visible - Winter's Grasp
Enemy: Nearest visible - Petrify
Enemy: target of warden - Attack
I would definitely not advise setting AOE spells (such as Fireball and Earthquake) in the tactics. The AI cannot be told to avoid your own allies.
Hope that helps. Good luck.