There are a lot of party members in Xenoblade Chronicles X, and seemingly all of them have special sidequests which require using them. Since only characters in your active party get experience, how can I quickly and efficiently level up all my party members? I don't have any trouble keeping Lin and Elma leveled as they're mandatory for the bulk of story missions, but the 10+ other characters all end up behind.
What I do is as follows:
Hop in a skell (preferably one with very heavy-hitting weapons such as HOMURA, G-BUSTER, and PHOENIX -- intergalactic quality if you can).
Go to FN 306 and fly to the northeast hex, and up the cliff.
Here, there's a level 70 saltat. It's very easy to kill repeatedly and grants a great amount of XP - I've been leveling up my characters relatively easily due to this.
That said, if that spot is too difficult for where you're at in the game, there are also level 51-53 saltats one hex NW (and around) of FN 106. The strategy here is the same - attempt to one-shot them, then finish them off in other ways.
The above strategy works. However in the end game (when you get the flight pack) there is a better site. Fast travel to noctilum and go to the divine roost ( 3 north most fun sites) plant probes then using skells fly to top of central plant thing ( I recommend all 4 team members have skells your character having a level 50 and the other level 30 heavy skells (Adamius and masters work and equipped with buster and snipers,explosive spare weapons and also if possible the phoenix back weapon ( makes the fight MUCH easier). Now use phoenix on the level 60 Blatta ( to start the fight use phoenix to trim enemy health, ( don't mind uncontrollable playing the level 90 tyrant is actually really weak (it's a oversized Blatta so shouldn do to much damage.) Use the explosive weapons to deal with the swarming level 60 Blatta and the if confident focus on the level 90 tyrant if you can't kill it don't worry just run as it can't follow. Then after the fight look at your exp bar, this fight should level you up immediately if you under level 55.
What I did was I bought a full set of EXP XII armor, which gave me a 120% EXP boost to everything I killed, so I didn't have to go out of the way to level up.