I feel like the difficulty increases way too quickly in a game of near-pure luck.
Take the slimes for example, the slimes appear on wave 4. Meaning you get 3 upgrades, the slimes appear in pair with 2 2-6 coins and +5 defenses, so they outnumber you, have the same amount of coin if you choose to get coin for all your upgrade, their max roll possible is above average, they have defense higher or equal to half of any of your coin roll. The only advantages you have over them is health but your base health is 30 and their health is 18, so not even equal to all their health combine.
If this is a game about skill then players can fail and get better and beat them easier next time but both upgrades and coin roll are rng meaning player's only able to choose the distributions of coin rolls, no matter how good you are at it, you simply cannot always beat the slimes if you got bad rng on upgrades and roll.
actually, slimes have are not that strong, stacking up your defense makes you immune from them, also do not take reroll, its useless, the expected value of all the coins are 5 so its better to have a 1 or 2 gamblers coin and have the rest as stable as possible (like a 4-6 or 5-5). and its a very easy game once you know the defense and attack stacks on the enemies. and in the end game the rng does not even affect the game because the odds even out when you have more 7~8 coins. it becomes more of a distribution game.