Okay, I re-read one of your posts, and I'll give my two cents.
Spoiler: (Show Spoiler)
I remember also being stuck at the Aeshma core for a while. My first time didn't go too badly, but that was because I actually didn't know that you had to go for the power lines; I just locked on to the first enemy in sight and didn't even consider the fact that there would be another one. But what I found was when I started attacking the power lines, I was dying WAY quicker than when I was just fighting against an infinitely-healing Aeshma core. It took me a while to realize that the power lines actually start doing spike damage; I wasn't completely sure that spike damage even existed in Xenoblade 2 since I hadn't seen any other enemies have it before that point, so I wasn't paying close attention to my health. What I personally did to overcome that was stop targeting it and build up specials and heal up by fighting against the core itself for a while, and when I had a special built up, I would just really quickly switch my target to the power lines, use the special, and then switch back. It took a few tries, sure, but it wasn't any real trouble once I figured out the trick.
If you tell me what you're stuck on now, I'd be more than happy to help--I was definitely NOT over-leveled for any portion of this game. I'm pretty sure for the majority of the time I was just around the same level as the boss, or even below them somewhat, and I can't think of a time I was over five levels above any boss creature. Of course, strategy in this game depends on what Blades you have, and I'm sure you don't have the exact same blades as I do, so there will be a limit to the advice I can give. But in general, racking up a bunch of elemental orbs and then using a huge chain attack to break them all and deal massive damage is generally the key to winning tough battles. Just like chain attacking in Xenoblade 1, where you could get insane amounts of topple time or deal huge amounts of damage by using the same type of attack in succession--it's the same concept, just a different implementation. Also, have you cleared Chapter 7? If you have, then you have pretty much everything you need to deal massive amounts of damage with Blade combos and chain attacks--if you have cleared it, then I'm sure you know what I mean.
I'll also add to this that I believe you said you were focusing on Nia, right? I won't lie, I never switched Rex out of the first slot in the party, so I can't say for sure, but the game might be a bit harder without Rex in the lead--especially when using Mythra, who is basically the best normal blade in the game with her specials and the buffs she gives you, as well as the way you can combine her effects with accessories to get great bonuses. If you aren't the one in control of what that character is doing in battle, I can see it becoming tougher, but I couldn't say for sure.
If you tell me what you're stuck on now, I'd be more than happy to help--I was definitely NOT over-leveled for any portion of this game. I'm pretty sure for the majority of the time I was just around the same level as the boss, or even below them somewhat, and I can't think of a time I was over five levels above any boss creature. Of course, strategy in this game depends on what Blades you have, and I'm sure you don't have the exact same blades as I do, so there will be a limit to the advice I can give. But in general, racking up a bunch of elemental orbs and then using a huge chain attack to break them all and deal massive damage is generally the key to winning tough battles. Just like chain attacking in Xenoblade 1, where you could get insane amounts of topple time or deal huge amounts of damage by using the same type of attack in succession--it's the same concept, just a different implementation. Also, have you cleared Chapter 7? If you have, then you have pretty much everything you need to deal massive amounts of damage with Blade combos and chain attacks--if you have cleared it, then I'm sure you know what I mean.
I'll also add to this that I believe you said you were focusing on Nia, right? I won't lie, I never switched Rex out of the first slot in the party, so I can't say for sure, but the game might be a bit harder without Rex in the lead--especially when using Mythra, who is basically the best normal blade in the game with her specials and the buffs she gives you, as well as the way you can combine her effects with accessories to get great bonuses. If you aren't the one in control of what that character is doing in battle, I can see it becoming tougher, but I couldn't say for sure.