I found Thunderbird in Zelda 2 the hardest, all the jumping and slashing while avoiding the fireballs. Without enough magic to use the Life Spell I got slaughtered alot before finally defeating him. I got sick of continuously going through the Great Palace only to get to Thunderbird with one or two lives left and dying.
The first time I finished the game I spent an afternoon in the forest near the Hidden Palace maxing out points to get extra lives. When your levels have been raised to 8 they max out and you get an extra life instead. I did this and took the Great Palace on with about 20 lives. From memory (this was about 15-20 years ago) I used about 9 lives to defeat both Thunderbird and Shadow Link. I found Shadow Link easier because I loved the one on one sword/shield fighting and got quite good at it. I beat SL the way you were supposed to, this was back in the day before the internet and walkthroughs revealed tips and tricks.
Still my most satisfying Zelda experience