Damn, surprised you thought final boss wasn't as hard as some others. Died sooooo many times on final boss haha. I definitey thought final boss upped the challenge from the other really hard bosses. Anyway, after you've gone through the game once it gets a lot easier, so you should definitely try out hard mode. Not saying hard won't be challenging, but like replaying normal mode after I beat it for the first time was soooo much easier. Hard mode is a good challenge after you've beat the game on normal.
And yeah, I tried to 100% the game right after I beat it for the first time and I also got everything except that one tank that requires just insane use of the shinespark. It takes some ridiculous shinespark skill to get that one.
I managed to do all the shinespark/speed booster challenges with some taking me quite a few attempts.
While they are challenging, I think you can manage with a little bit more dedication.
Well I got everything except the one super hard one. I feel no desire to waste a bunch of my time just to stay I got the last item in the game.