I found the difficulty a breeze. The sweet spot for me was world 3. By world 4, enough of the various characters were deep into their skill trees that nothing posed a challenge until the final boss. The post game missions were difficult even with the maximum number of upgrades with ultimate challenge 3 being my favorite mission of the game.
Once available, I ran Mario, Rabbid Luigi and Luigi almost every mission. Rabbid Peach was a good substitute for Rabbid Luigi in escort missions. Rabbid Mario was great for missions were you could capture a group of enemies clumped up. I wanted to like Yoshi/Rabbid Yoshi better, but neither was strong enough to stand up to the utility, movement and damage the rest of the cast provided.
I am in the 10% that find this game hard. Struggled on 2-8 and now have wiped twice on the World Two boss. I am enjoying going back to the challenges though. Even resorted to looking at general tactics online, so have been using the heck out of R.Luigi’s Vamp etc, but I still just get smacked around.
The biggest challenge in world 4 for me was finding the energy to push through the overworld to get to the battles. In the article though it mentions the issue of having characters on low health, I always solved that by having rabbid luigi put vampirism on someone, then having the others shoot that person. His world 4 unlocks don’t have the vampirism status effect, but his movement + world 3 weapons do