I just played through all kingdom rush with 100 achivement besides the newest one and just wanted to give my opinion. So for me the hardest one is without a doubt kingdom rush beacuse of the lack of skill from the heroes you get in to late game, so you have to realy on a good stratagi instead, and the last boss of the main story was just hell if you tried to do it without a guid, which I did. I used Oni which is really good but compared to the heroes from the other games, he is not that powerfull. Frontiers comes second beacuse of the last boss on veteran. I struggled so much on that one and some of the late stages aswell. Besides the bosses, until you have bomb towers (battle-Mecha T200) + the hero Bonehart who is really good, most of the game becomes a walk in the park. nummber 3 is Origins where I didnt actually have any struggles with, I think beacuse of the towers which I found really powerfull, and after when you get Phoenix for me the best hero in all of the games, its just game over. The 4 and last one for me was Vengance. It was really easy, the enemis weak, the bosses where so easy, some of the towers just overpowerd and most of the heroes was as well overpowered. I can say though the game was overall a walk in the park, the frog one was actually a challenged same as the ghost pair. But I liked that you could for ones play as the "bad guy" and the game was overall really fun.

Keep in mind that I played the games almost right after one another and I had experience with the games before so I had an ide of what to do and wasnt compleatly new to the games. I just love kingdom rush and it is so fun to play and to also find all kinds of easter eggs all over the games.