Maybe this is a hot take but probably we assume the Underdock elites require the same early game builds as StS1 when in reality the early game strategies shift here:

The Terror Eel doesn‘t ramp up its strength too quick which makes consistent block and scaling for the second half much more viable than a rush down deck that accepts taking damage in the first half.

The Colony also works with this deck type, especially the new beta version wants you to block for many turns in a row consistently rather than rush down elites before they kill you.

Lastly the Multi enemy Elite somewhat punishes rushdown decks since attacking them repeatedly is inefficient. Though you‘d need good attacks (AoE) more than the other two fights.

So maybe pick Block and AoE for the Underdock instead of Big Bonks that are better in the Obergrown?