in my second playthrough (new game+), i have FINALLY complete all side quest, level up to lv99, found all secret area and collectables.

i have 3 superbosses left to defeat, and few heart-to-heart (all my party members' affinity are at max, just lazy to do it)

now that i almost done what i want to do in this game, how do you guys actually define 100% this game? most of my skill are not max yet (damnnnnnnnnnnnnn those rare skill bookk!!!!!!), i was thinking about finding all the skill book but it is almost impossible -.-

Xenoblade is quite a massive game.

Getting all of things including those skill books probably do classify towards 100%

100% means exactly that. You have to have everything in the game, and have completed all missions. You should be able to do that before Dec.

I think Xenoblade side quests which are given to you by not-named NPCs are meant only to level up. So I would define 100% this way: beating the game, completing the affinity chart (with the named NPCs side quests), beating the bosses (with super-bosses and optional bosses as well), and heart-to-heart moments. That's enough for me.

i've finished the game on the single playthrough, defeat all superbosses even without using that new weapon for new game+

completed heart to heart, all areas are shown.  there is a catch, are you trying to complete the trials and achievements as well?  and the affinity coins can be up to 999, if you are going to play and finish it several times. are affinity is really hard to complete but atleast on par with most sites or go to different path about certain quests which gives you different affinities.  there are several NPCs that you may miss, but if you are a completionist, you may wanted to push the limits about area affinity. 

about the party member affinity, there are a few unique monsters that has a sleep skill that and you have sleep resist, easy to grind affinity hearts. :)

i've unlock the new game+ but didn't played that much since I've mostly tried to complete the game on my 1st playthrough.  i wouldn't know if it is 100% as it is really hard to complete the trial list.

It means beating the game without doing a single side quest. It makes the game 100% fun.

Your description matches what 100% means

100% isn't exactly the same across all games and can be a bit subjective tbh, most cases it's get all items, quests, skills etc... But some games it's not possible to do so in all categories so 100% is down to the individual game itself and what's possible in it.

Just reaching Lv. 99 was good enough for me.

I always defined doing 100% of a game as completing all the content (so beating every super boss, doing all sidequests and completing the main scenario). Getting your character to max lv or leveling up your skills I never count.