It’s weird. If I’m in a party and a party member dies, my DPS doesn’t suffer. If my pet dies, my DPS tanks. It’s almost like the pet is built into my class, I control it, and I’m playing solo.
What you probably meant is that Hunters (and warlocks) are the DPS that can solo Torghast effectively. While Mages are pretty squishy, and rely heavily on being careful, CC’s, and downtime between packs of enemies.