I want to build a shaman-occultist focused on health regen and pets. The idea is to shield a friend playing a caster who deals damage. Throw up lots of pets and take a lot of damage. To figure out how to do that, I looked at the skills for each class and analyzed them in relation to this goal.

Each class has a few skill lines that are highly relevant and a lot more that are more marginal. I break these down in separate sections on Shaman skills and Occultist skills below. I'll come back to the overall build in the Postscript.

Skill names are in bold the first time they are used.

Shaman skills

The Shaman has a few highly relevant skills for this build. It also has a few marginal ones.

The highly relevant skills:

1. Mogdrogen's Pact line. Pretty great buffs to health regen and energy regen, especially with Heart of the Wild, and decent resistance and retaliation damage buffs with Oak Skin.
2. Wendigo Totem line. More health regen. Also deals vitality damage to enemies in range. Blood Pact adds a short-range (5 meter) buff to the totem to convert your attack damage into health. It's not specified, but sounds like this should also apply to any allies in range. This also increases vitality damage, bleeding, and vitality decay dealt to enemies, as well as duration of bleeding and vitality decay.
3. Summon Briarthorn line. The Briarthorn is a pet. It hits things. It's goo at that. Ground Slam makes it more threatening to enemies which is great for defending the caster. Emboldening Presence makes the Briarthorn buff nearby allies' retaliation damage, all damage, and resistances (15 meters).
4. Wind Devil line. The wind devil is a short-lived (18 second) pet. You can keep three going at a time if you keep summoning them. With Raging Tempest they can slow enemies and slash those enemies' elemental resistances, which could be a nice way to support the arcanist. Maelstrom increases (summon?) duration, targets, and attack radius, as well as just buffing the devils' damage.
5. Storm Totem line. The storm totem is another fixed-time (14 second) pet. Again, limit three at a time. They deal lightning and electrocute (lightning DOT) damage. With Corrupted Storm you can convert the lightning damage to vitality damage. (Does that also convert electrocute to vitality decay?)

Then there are the more marginal skills:

1. The Brute Force line. The base skill gives a little bonus health which is nice but not amazing; otherwise it's just damage. Feral Hunger triggers on 26% of attacks, hitting at most 5 targets and healing for 33% of attack damage, which scales with main hand damage. This is decent healing. Upheaval is just damage.
2. The Primal Strike line. It's just damage. Not bad for fighting without the arcanist's damage, but doesn't directly help with health regen or pets.
3. The max rank skills. Conjure Primal Spirit gives a short-lived (20 second base?) pet which deals physical and bleeding damage as well as piercing and poison damage. The cooldown is long at 45 seconds. Primal Bond is a toggle and buffs pets' damage by a percent regardless of type as well as their crit damage, and it gives them flat bleeding damage. It also buffs the shaman with "damage absorption" (is that % of incoming damage converted to healing? nullified? at what point int the calculation?) and increased damage and DOT. Stormcaller's Pact buffs various damage stats. The only relevant bonus is reduced stun duration but there are plenty of other sources for that.
4. The Savagery line includes Tenacity of the Boar which seems to increase health regen by a percentage (up to 100%) and increases slow resistance. Unclear if you need to use Savagery to activate those bonuses, but the health regen would be nice for this build. Might of the Boar gives a bit of physical resistance but its main benefit is a percent increase to "total damage." Storm Touched is just damage.
5. Devouring Swarm is nice early on for converting damage to health, but as far as I know it doesn't scale, so it becomes useless later on.
6. Grasping Vines is a neat debuff that slows enemies. It could be useful for preventing an enemy from reaching the caster. 12 points is a lot of points for this, though. Entangling Vines doubles down on this and doesn't seem worth it for this build.

Occultist

Highly relevant:

1. Blood of Dreeg line. Restores health within a radius and applies a flat health regen bonus to self and allies. With Aspect of the Guardian it can also buff retaliation damage, poison/acid damage, and vitality (decay) damage, together with physical reistance and poison/acid resistance. The base skill requires just 15 points in the Occultist class while the upgrade requires 32.
2. Summon Familiar line. Does a little damage. With Mend Flesh the raven can heal allies in a 3.5m (?) range for a percent plus flat amount. Storm Spirit buffs allies' elemental damage in a 20m range. Lightning Strike is just damage and a bit of stun. Getting all three requires putting 25 points into Occultist, but to get just the first two you only need to put 15 points into it.
3. Summon Hellhound line. It's a big dog that can wreck shit up. Looks like not a bad alternative to the Brierthorn, but takes too many skillpoints to run both. Ember Claw generally increases damage and threat. Hellfire buffs allies' fire, chaos, and burn damage as well as granting them flat fire retaliation damage. Infernal Breath is another hellhound damage ability. This buff requires putting 32 points into the class itself which is a bit much however.

Marginal:

1. Bonds of Bysmiel line. Makes pets healthier and regenerate energy faster so they can use abilities. This is deep into the Occultist class, however, requiring 32 points for the base skill. However, it costs energy over time in addition to the reserve. Manipulation makes them faster and deal more damage, but requires putting 50 points into Occultist. Overall this line does not make sense when using Occultist as a secondary class.
2. Sigil of Consumption line. Looks like Devouring Swarm but buried deeper in the tree.
3. Solael's Witchfire line. Just an attack.
4. Dreeg's Evil Eye line. These mostly do damage and debuffs, i.e. nothing we want.
5. Curse of Frailty line. More debuffs.
6. Bloody Pox line. Damage and debuffs.
7. The loner skills Doom Bolt, for damage, and Possession, a damage-focused buff.

Postscript

I analyzed this based mostly on (1) is it a pet skill, or (2) does it improve health regen, or (3) does it improve some defensive stat. I didn't prioritize specific defensive stats other than health regen.

As far as an overall build, I plan to focus heavily on Briarthorn and Mogdrogen's Pact in combination with Wendigo Totem. For now I'm also using Feral Hunger an Primal Strike though I may respec out of those later to get more points for pets — wind devils, for example. I'll take Primal Bond and I might also get Conjure Primal Spirit if I have points left over. I plan to put just 15 points into Occultist to unlock the best raven/Familiar abilities together with Blood of Dreeg. That's not using the secondary class much, but that seems fine right now. The Shaman class has plenty of pets and other lines to spend skill points on.