Spirit Walker is a component of Trigger Team so he's recommended. Witch Doctor is more for PvP but not really optimal with the current meta.
Sven Berserker can act as your main Damage Dealer. Put a Kahuna Spirit Walker behind him, a lifesteal hero (Gladiator Drake/Samurai Miyu/Leon Blood Knight/Darrion Blood Knight) beside him to trigger Kahuna's aura) and a Vampiric Rune for maximum triggers. It's recommended you leave one slot for Sven's summon if you can.
After that everything else is optional. Another lifesteal hero beside him perhaps or maybe a Samurai Shizu or Witch Doctor Matilda. There are a lot of ways to maximize damage.
You can also take another route and use Dragoon Drake as your main Damage Dealer. Surround him with Kahuna and some lifesteal heroes. I can't really tell how they compare tbh I don't have one but I've seen someone else use him.
With a good Ally Sven Berserker w/ or w/o Vampiric Rune, Kahuna Spirit Walker, Drake Gladiator I reach up to 1.5 million damage per key. I have my own Sven but his gear isn't that good so I get up to 800k damage per key.
I have vampire rune, refresh,dodging, resillience and double strike. what should I choose ? I don't know to choose rune for hero in my squad