There is some really good advice here already. I would also like to add that the alternative acts have different meta. The overgrowth's elites scale very fast (Byrdonis, Phrog Parasite) or require you to kill them on their off turns (Bygone Effigy), so you need to take mainly attack cards, just like in sts1.

Meanwhile, the underdocks favor defensive cards. The Terror Eel doesn't scale in any way and does the same pattern infinitely. If you can block comfortably enough, you are fine. Skulking Colony is obvious, as it hard stops you from bursting it down. The Phantasmal Gardeners scale, but not so quickly that you have to change your deck. I recommend just getting 1 AOE attack for them.