one more thing. The randomness of the shop system adds realism to the game. In life you will not always be able to get what you want when you want it. In this game you've cut your self off from major trade routs what comes to you is what comes to you. The difference is that great heroes use the "cards they're dealt" to the fullest. If you can't do this then note my previous comment.
This game is hard, because it's meant to be. The fact of the matter is that even with difficulty settings easy is to hard for some and hard is to easy for others. So games are created extra hard and extra easy for those skill sets so that every one can find a challenging, but not impossible game. Their are plenty of "easy" tower defense games you could be playing, If you just don't have the skills for this one.
Tip for those of you just starting this game for the first time start one in save slot 3 insted of one select casual mode and insted of the job you want like hunter or sherif select the opposit of it do this just to earn some medals so the game you realy want starts out with the bonuses of the medals from this scrap game.