At the moment, I am trying to clear ascension 20.

My chance of winning the game up to ascension 5 is > 50%.

IMHO StS gives the player bad habits in the lower difficulties (and the difficult spike when reaching the heart is not that great).

Some tips stolen from better players than me:

• You really have to play optimal and think about every card to minimize life loss (This could also mean taking a hit early and kill off an enemy faster vs. blocking an early hit and taking massive damage later.)
• The bosses especially can be seen as problem to solve. Look ahead, do you have the right card(s) in the deck to solve the problem?
• The first hall is mostly about about up front damage, in the second you will need some area of effect damage etc.
• Before adding a card to your deck, answer the following questions:
  • How high is the chance I can even play this card? (Example: You have 3 energy and a card costs 2 Energy. If you have another 2 Energy card, you know increase your chance of a dead draw, because you can not play both if they appear in the same hand)
  • Does this card has any synergy with the cards I already have in my deck or with any artifacts?
  • Does this card solve a problem (e.g. boss or enemy) I have? IMHO all the generic advice is not wrong (like having as little cards as is possible), but the point about playing the higher ascensions is really more seeing 'the whole' instead of focusing on one aspect. For example, if you have Corruption in a bottle and Dark Embrace, all of a sudden you want to have as many skills a possible... OTOH, if you have two Dropkicks, you want everything which destroys cards to have an endless Dropkick-Engine as soon as possible.
• Take care of immediate problems you know you will face (like the boss of the current hall), instead of speculating on card combinations which might or might not show up in the future. (Exception to the speculation rule: Iron Clad has Limit Break, and it is total reasonable to expect to find some strength boost as an Iron Clad, so LimitBreak is usually the one card which I never think about picking up)

tldr: Picking your strategy and adding/removing of cards must be seen in the context of artifacts, energy and the bosses you will see. Optimize for your next known problem, instead of betting in cards to become available. You can have 1-2 cards for special situations or as speculation, but adding for example another attack card if you already have enough of attack, simply doesn't solve a problem or makes your deck stronger.