My husband is playing the witch dr on xbox one. Hes on t12 and paragon 490.
He has on Jade Armor set. Uses Haunt, soul harvest, posion darts, zombie dogs, corps spiders and gargantuan. Pasives are bad medicine, creeping death, jungle fortitude and swampland atlunement.
We have tried switching stuff up but these havekept him alive the most yet during greater rifts and nephalem rifts he is dying non stop. During bounties hes fine but any rift obliterates him… Any advice appreciated 
I’m not a WD expert but Jade is a DoT build. It is very rotation specific. They talk about it here, note you have to delete the ~ from the below for it to work.
~https://www.icy-veins.com/d3/jade-harvester-witch-doctor-skills-and-runes~
Pironado to group, then
Locust Storm, then
Haunt, then
Spirit walk into the the entire mess of enemies, then
Soul Harvest to kill and gain armor for each enemy affected with the Lakuma bracers.
You live, things die.
That site does have links to a pet build, or a Poisen darts build if one of those styles is what you would like to play.
I’m not expert of WD, but have played Jade build for sometime.
You may try Horrify(Frightening Aspect) skill which increase your armor. With some Cooldown Reduction, you can cast it all the time.
My zdogs use rune Leeching Beasts to recover my health.
I also use Uhkapian Serpent mojo to offload damage to zdogs,
Unity to offload damage to follower (if solo).
Sacred Harvester - very important for Jade build, IMO.
Lakumba’s Ornament bracers - damage reduction
Use Amethysts on chest and pants to increase life.
Endless Walk set also help to reduce damage.
My S17 season Jade build: https://www.d3planner.com/563061696
Poison darts, zombie dogs, corpse spiders and Gargantuan are not skills used in a Jade Harvester build - they are used in Helltooths, Arachyrs and Zunimassa. Use Haunt, Locust Swarm, Piranhado, Soul harvest. I use Horrify and Spirit Walk also. Get a Sacred Harvester dagger and a Henri’s Perquisition mojo. Also Lakumbas Ornaments bracers… As said above, go to icy-veins and they give a good overview and break down of the gear and play style.
p.s take a peek at my seasonal WD and that might give some ideas.