Immortal Spelunker

Select a delve variant that minimizes forced combat or has an easier final boss. Recommended options are: The Underkeep on Runaway Evolution (no traditional final boss — three Nerubians patrol set paths and can be fought individually with less HP than standard bosses), Mycomancer Cavern on The Great Scavenger Hunt or Fungal Folly on Explorer's Competition (both feature non-combat objectives), and The Dread Pit on Lost Gems (many skippable packs). Avoid The Underkeep on Torture Victims, The Spiral Weave on Tortured Hostages or Strange Disturbances, and Earthcrawl Mines on Kidnapped variants, as their final bosses are significantly harder.

On Mycomancer Cavern, the end boss appears at the start of the delve, so dying and resetting mid-run is not a concern; after defeating the boss, you only need to collect fuel and kill 4 enemies plus some trash, and the most dangerous packs can be bypassed entirely due to the delve's layout.

Bring Brann Bronzebeard specced as a healer (level 30 recommended) to sustain through unavoidable damage. On The Waterworks with Captured Engineers, the boss Waxface deals mostly avoidable damage, but for anything unavoidable, heal through it and use personal cooldowns.

This achievement does not require soloing — running with other players is a valid strategy. Solo attempts are not balanced equally across all classes and specs, so some class and spec combinations may find certain delve variants extremely difficult or effectively impossible.

As a Shadow Priest, talent into Dominate Mind and look for an Ascended Webfriar enemy inside the delve. Their Web Blast ability scales very strongly at higher tiers and can solo most mobs — watch the cast carefully while controlling the mob.