So I have Iguana Tweaks, installed with OreSpawn, Twilight Forest, Thaumcraft, And Zelda Sword Skils - And I've come across this issue.. almost any mob that has "Armor" ... Basically becomes unkillable. - I was wondering if this was possibly caused by Iguana tweaks

So after some testing on my own, Iguana Tweaks is causing the issue with making mobs with over a certain point of armor (12 seems to be the magic point on most monsters), they basically become unkillable. You can't do any damage to them. this is also true with players over 18 points of armor. Is there anyway a config could be made to turn this option off? Or maybe just remove this, if it is a bug.

Nope, ITT does nothing of the like.

So then it must be a conflict when it's paired with another mod. Cause, I didn't have this issue when I removed ITT. I'll look around some more, if you like, I can also post a list of my mods, and maybe you'd have some sort of idea, what might be causing it.

What were you using to hit the enemies? If it were non-tcon tools you might've used a disabled weapon. Otherwise it's another mod causing it nonetheless.