Because Zain is Persistant, it makes him very difficult to kill to fight. For first he launches four fireballs instead of three, so that isn't very good. In addition, the Persistant buff comes also in as an aura, making all enemies in its range get resurrected once after they're killed, making the horde much harder to beat. Fortunately, Zain is the only resurrector on the level, and nearly all enemies need to get close to the hero, before they can attack.
Viper is good because his Alpha Variant ability, Virulent Vines, good for picking off Minions. Also because of his Squad Ability, Thornado. In addition, any enemy he damages becomes Poisoned, because of his Passive Ability, Toxicity.
Meditron, on the other hand is a good healer with his Squad Ability, Reconstruct, unfortunately, it is a channeled ability, meaning he has to remain still, making him vulnerable. But his Passive Ability Sentry Drone would be at least a small asset (better SOME defense, than NONE), while whenever he would be hit by Zain or an other Terrosaur, he could simply get close to a Darkspore (never mind if it's a Minion or Lieutenant or even Zain himself), then just hit Syndrome Shift, and knock'em with Kinetic Wave.
Magnos will be useful for all his Abilities. His Shooting Star will be useful against Zain, as the Plasma Captain usually stays on the back, making the damage even badder for him. His Gravity Well and Kinetic Wave combo will simply slaughter all Minions in range and badly hurt the Lieutenants. However be warned, as Zain and all other enemies will keep shooting at you, as well as the Persistance Aura.
However, a combination of Skar Delta and Viper Alpha is definitely the best choice. First switch to Viper (unless you're already using him), activate Shadow Cloak, then kill everyone with Virulent Vines. Remember to target Zain, since you want him to receive the most damage. If anyone survives, switch to Skar, Shadow Cloak again, then pick off the survivors with Affliction Bolt. If anyone still survives, just get rid if them one by one.
- His title "The Hellfire Death" is a reference to his fireball attack, burn debuff and probably near unkillability, as (probably) many players fighting him for the first time have lost.