Okay, well a little update.
The reason Tidus was hitting so low was because of the enemies defence, he still isn't hitting over 1,000 on the weaker enemies though. The Omega Ruins were going great until the enemies suddenly became harder and I died before I could get to the next Save Point, it even blocked me from fleeing (WTF).
I've decided to go ahead and kill Sin, if I want to grind I'll come back to an earlier save file.
...As for killing Sin, I am still extremely under-levelled. My main questions are:
Are there any places or towns I should visit before I go and kill Sin?
What items should I stock up or find before the battle?
What are some useful abilities I should use on my armour / weapons (sensor etc)?
Are there any good places where someone with 2,000/3,000 health could grind?