Well, by now you've probably noticed the monsters below Treno's weapons shop.  I can give you advice for Disc 4's monster, the Behemoth.  
 
We all know that you encounter the Behemoth in Memoria, but that's with a full party of four.  Fighting a Behemoth alone is INCREDIBLY hard.  Now obviously I'm talking about RPGamers like myself.  I don't have level 99 characters with the ultimate weapons yet.  If you are one of those kinds of RPGamers, then this guide doesn't apply to you.
 
First, you'll want your candidate to be a strong physical fighter.  The three top picks are Zidane, Steiner, and Amarant.  You can use Freya, but she just doesn't measure up.
 
Next, unequip ALL your abilities and re-equip these:
 
1. Auto-Haste
2. Auto-Regen
3. Auto-Potion
4. Beast Killer
5. MP Attack
6. High Tide
7. Counter
 
Those are, of course, in order of priority, so follow those strictly.  You may think I'm stupid for listing High Tide so low, but I beat him without going into Trance, and Zidane was only at level 44!
 
For the battle, follow these guidelines:
 
1. Just use your initial attack only!
2. Don't use an Elixir unless you're really hurting.  With both Auto-Haste and Auto-Regen, you can regain about 800 HP in a minute! 
3. I can't stress enough the importance of Beast Killer and MP Attack!  With both of these, Zidane did 6,000 damage with just an Orichalcon!
 
Well, I hope that made the monster battle a little easier for you guys. See ya!