Damn, I'm either REALLY rusty at Kingdom Rush Vengeance or the first of the post-game levels is just a pain in the ass.
Haven't played the game much since beating it originally, but the devs are always great about supporting their games and they've released at least two new batches of levels since then.
I've only played the first of the new fish people levels and I've only been able to get 2/3 stars in it even on casual. It just has so many bullshit moments in it, a new enemies that lunge huge distances to hit your barrack units, pretty much all of the new enemies only being weak to magic, support enemies that give shields to the already super tough enemies, and like 3/4 of the way through they throw a huge wave of flying enemies at you and spam the huge health big enemies.
I love Kingdom Rush, but playing Vengeance does make me wish they hadn't changed the way towers work. Having to switch builds constantly to try new things is a pain and half the time I dont even feel like I'm making any forward strides on a new strategy. I watched videos on YouTube for the level and most of them only use like 2 towers, one of the even managed to win on hard with ONE tower type.