Faction: Play as Necrophage.

Units: I recommend to have at least 8 Infestors (the only unit with the Infectious ability).

Upgrade path: Larva → Spitter → Corrupter → Infestor

Total resources for 8 Infestors: 800 Corpses + 80 Titanium + 4,000 Dust (see upgrade path).

Target Army: Find an enemy with at least 6 units (minor factions will do as well)

To upgrade from Larva → Infestor you will need to research the following technologies, marked with bold below (you may easily find them using the search functionality in the Research panel):

• Larva → Spitter: No technology required. (Cost: 20 Corpses.)
  
• Spitter → Corrupter: Razor Mandibles. (Cost: 30 Corpses + 10 Titanium.)
  
• Corrupter → Infestor: Chitinous Plating. (Cost: 50 Corpses + 500 Dust.)

Army 1: 4 Infestors + 2 units with high health/defense (for example 1 Hero + 1 durable unit, or 2 Heroes).

Army 2: 4 Infestors + 2 units with high health/defense (for example 1 Hero + 1 durable unit, or 2 Heroes).

Heroes/durable units are not there to kill, but to soak damage and weaken enemies for Infestors.

Optional: Defensive items/skills on heroes to help stall the fight.

1. Engage manually - do not auto-resolve.
2. Focus fire with Infestors only for last hits.

• Use heroes/durable units to damage or tie up enemies.
  
• Always ensure the killing blow comes from an Infestor so the Infectious ability triggers.

3. Spread out the kills across Infestors so each one has a chance to infect.
4. Protect Infestors - keep them at range, use tanky unit/hero to block choke points.

Weaken first: Use non-Infestor units to bring enemy health low, then finish with Infestor fire.

Defensive anchor: A durable frontline unit helps buy time for controlled kills.

Enemy choice: Armies with 6-unit stacks that include mainly infantry units are predictable and not overly mobile.

Save before battle: If possible, save before engaging so you can retry if kills don’t line up right.

Last kill: after the last blow, when the 6th Larva unit is created, the battle will automatically end, however you will get the notification of the achievement almost instantly.

Hope this helps...