Grind. you can farm arena or go back to older maps. Wan you get to the first wall like map 4 or so go back to map 1 get 3 stars on all level than move on to the next map . Do research look at the combos and go for marijuana first than bomb. Marijuana will do more damage and heal cards and bomb will damage cards nest to it. Wan you get to higher arena levels a box will give more cash as for viagra map 5 is the sweet spot for now you get 385 at a 60% from 5 to 10 you get 600 more cash from 10 but it's a lot harder to win. At the end of the day if you don't pay you got to grind.
Viewing post in Is there no way to farm previous difficulty levels?
No that's way you need to do arena and research. You're not going to have a easy time at the first wall. Every difficulty level will give you more cash/viagra but the first wall will take time to pass out side of buying packs. Grinding boxes for cash to do research and hope you can get a good combo to lock the first spot on the map is the only way. The trainer is a good card to use at the start cuz butt plug and marijuana.
p.s. The rarity of the card will add abilities to the combo: common=none/ rare=first character ability/ epic+=first character+item ability so on need to power up or keep common cards unless you're collecting all the cards