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If you're still interested in the cave of trials. Here's the particulars for it. The first part will have 11 monsters, with the lowest level being 27 and the highest possible at 35. This would be a good way for you to train and beef up the skills you have.
Area 2 will have 9 enemies, the lowest at 35 and the strongest at 50. Each time you take these critters out, remember to use abilities as much as you can. Level them up so you can deal stronger damages when you get to Mayonnaise Harbor.
The third cave will be significantly more difficult, 18 enemies each with levels higher than level 70. Again, deal with them by using your skills so you get to gain experiences for your skills as well as level up faster. Now the fourth cave will probably be the last cave you can finish without your 3 other characters. Even then it will be quite a challenge. 13 whooping monsters with levels at 70 -90. Still your 3 supporting characters will probably be able to finish them off, although it might take longer and harder than with your best three.
Didn't I say the last area was impossible without your other characters? Here's why... You'll have 15 enemies, with the weakest at 110 and the strongest at level 500. If you want to, which is actually what I did - have your characters focus on killing one monster (the zombie) at a time, with a decoy ready to flee once the going gets tough. Just keep healing and then going back there. it will be a fast level up process but kinda tedious what with going back to the world map and all.