Oh yeah, that big mess of a Slug can be a bit tricky to beat. Just make sure you're well-stocked up on plenty of Dispel Herbs. It gets rid off all negative status effects. I suppose I could give you a couple of hints here, and there's no need for level grinding. I beat it at lvl 8/9 with Alys, 8 with Chaz, 7 with Hahn and 6 with Gryz.
- First and foremost, make sure Alys is equipped with a sword, otherwise you can't use her OD.
- Buff your chars first round and have Hahn cast his Gelun on the Fract Ooze to greatly reduce it's attack.
- Do not let Alys and Chaz use their Crosscuts one after the other!
- Make sure they are buffed with both Shift and Vision for max damage. Al can pull off nearly 190+ damage like this. Same with Chaz.
- If you get hit with that horrible Bad Smell ability, get rid of the Poison immediately. It takes away 10% of your character's HP for every turn gone.
- Try to keep all of them alive.

You can apply this strategy to most bosses.
Hope that helps.