Few questions about Tales of Symphonia

I'm not sure what part your in, the middle or the end, I never had any problems with leveling up, but I would just go to the most recent place you were, with monsters your adept at fighting.
About niflheim, it's only when you beat the game you can access it, if I remember correctly.

I'm about to enter the Tower of Salvation for the very first time. I guess I'll go back to the Tower of Mana and grind there, then.

I've already beat the game twice, on my third playthrough, but it's been a while since I've actually played it.