I want to buy this game but I'm afraid it'll get too repetitive and boring fast...
I was afraid of that too. Continuing on from what Jake. said, I got bored of Smash 4 due to its repetitive gameplay, so it didn't hold my attention much after about 10 hours of playing. Hyrule Warriors might be the same gameplay over and over, but it's still interesting to me to play after 70 hours of playing. The fact that I'm a fan of hack and slash titles might help with that.
The game has Legend (story) and Adventure Modes, with Adventure Mode giving special rules for battles. Those special rules alone make the missions interesting to play, and thus less repetitive. The DLC only adds to that. More characters, more Adventure Maps (with more special rules), and a bit more story.
I would recommend it if you enjoy action games and Zelda games in general. The game manages to hold your attention.
I'm having trouble with the adventure mide mission where you have to defeat all enemies as fi. I can get through the first 2 sets fine, but I have a ton of trouble with the imprisoned. Any tips?
lol, I remember that one being awful. I still only have a C rank on it since I want to level up Fi more before I attempt it again.
Take out the Beamos with bombs and try to learn The Imprisoned's tells. He has very large wind-ups for his attacks. Also, near the end of his health bar, him stomping around will create blasts that hurt you. Fi doesn't have a lot of ranged attacks that can help with that sadly.
If you're having too much trouble with it even after that, I'd suggest moving on from it for a while, and leveling up Fi in other missions. I don't believe not doing it locks out parts of the map, as you can get there from other directions.