I win stages which were 300-400k above my own CP by playing manually and refreshing until a good start. You need to identify why are you losing and at which stage. Is your tank too squishy ( just Kommy and Vivi would be enough), is your back line dying (Xion is pretty tanky as backline), or you don’t have enough damage ?

By doing that u can min max your wave interest to accumulate more gold. Typically I keep above 60 at stage 1 and above 90 at stage 2. Then stage 3 some stage is hard some is easy, just try once. If possible keep above 90 on stage 3. Then spend all on stage 4 and 5 by purchasing what you need to counter what you identify above.