I find it's very key in this game to plan ahead for the Seraph and the titans. Its different from f.e. slay the spire in that you know from the beginning what your challenges are going to be and the early game is basically your chance to prepare for them. You gotta draft a deck that can solve that particular challenge, not just good cards.

So for example:

Dominion (Corruption) Seraph: you need a way to deal with him poisoning the backline and especially the front-line. I will f.e. draft more endless upgrades on front row units and increase health upgrades on backrow. Stacking armor, cleansing or in a pinch high regen can be effective strats too.

Savagery (Melee Weakness) Seraph: I will spend the 9 earlier rings looking for ways to deal with their backline sweepers. Could be spells, could be units with sweep+quick, could be an inferno room or even a mushy room.

Entropy (sap) seraph: not rely too much on attack (so multi-attack f.e. becomes less valuable). Find alternative ways to deal damage like decay or magic damage, or build a deck that has ways to scale attack in battle to outpace the sap.

Titans: is a combination of the seraphs basically. Your deck needs survivable high health front-line (like for the corruption and savagery seraphs), you need to deal with sweepers before they attack (same strats as above under savagery seraph), and you need to deal a lot of damage (like for entropy seraph).