I read ghelas tip on deck building and I thought I could put my 2 cents in.
I personally like to bring in two damage reducer cards, and I try to make them my lower star cards. If your strategy is to win 3/4 rounds you better be a god at perfect pilling. By bringing in damage reducers, you only need to win 2/4 rounds, which is a lot more manageable.
By also taking lower star DR cards, it allows you to use your bigger star cards as heavy offense.
This was the traditional way to build decks, and it has changed a lot with the addition of solid 4* blockers, and the ban of 2* blockers from most formats.
Tip 2: Look for clan synergy.
I like to recommend pairing clans. My typical format is to use one clan that requires a lot of pills to win rounds: power manipulation clans, or damage clans. With clans that don't need a lot of pills.
This ends up looking something like this:
Ulu Water with Huracan
Berzerk with sentinel
Frozn and Pussycats
Where I allocate the majority of stars to the pill hungry clan, to really get a nasty card in. Fiddler, Daddy Jones, any 5 star Frozn.
As for Rowdy's comment on activator cards, it is generally a good tip.
But you can use it to your advantage. My current EFC deck that I'm top 50 with has 5/8 cards that need a requirement to be activated, with two other cards being equalizers. If I were to always play my deck optimizing my abilities, I would be playing predictably every single round. So I don't. I use reprisal cards on courage, I will use a growth card in round 2, I will throw an equalizer card at a two star. Playing unpredictably gives me large pill leads, and I use what would be a weakness as a core strength to the deck.
I disagree with Matrix 1st tip. The goal is to ALWAYS win with 2 rounds and you have to build your deck so you have enough damage/life gap even with your worst hand. I did fine with decks where the 4* and 5* were deffensive like Geoffrey and Aviria Ld and the 3* and 2* were offensive. I don't think it's important where you place your offensive cards as long as you have enough life gap to win most matches with only 2 rounds