Sewer Rats cast under the opponent's site on turn 1 or 2 if possible, or Muck Lampreys under your most forward site on turn 2. Then Devil's Egg or Nadir Seed or Blightstone into the hands of your underground critters on turn 3. Alternate play of Orm Infestor for the same effect.

Spells in hand don't matter on initial draw, prioritize hard mulligan for a defensive site (The Void, Bottomless Pit, Great Wall, etc.). This is a greedy build, so mulligan to get threshold defensive site (Gnome Hollows, River Rapids, Perilous Bridge, Wuthering Heights, Troll Bridge, etc.). If they're playing Sorcerer then hard mulligan for Garden of Eden. Always keep Garden of Eden (two key threshold, and we focus on card selection, not draw). Collection contains 1 Troll Bridge and 1 Gnome Hollows to boost odds to draw from 3 to 4 with Ersatz Platz.

This has a flexible package with 3 Toolbox and 4 Common Sense so you can tutor up answers and finishers. There is 1 copy of each key ordinary card in main and in collection.

Use Seer scry on Atlas until your defensive suite and threshold are online. Then Seer scry for key non-ordinary spells and tutors.

There may be other lines of play that prioritize removal over the primary life loss game plan, but you definitely need to get life loss online ASAP due to the grindy nature of this strategy.

Use Feast for Crows for whatever tech they have against underground such as Lava Flow or Ground Slam, etc., or wolves if they are wolves.

Blightstone and Acid Rain for any effects that silence your defensive sites.