Infern Isle,Danger level C. Becomes Danger level unknown later on in the game.
Infern Isle is home to many dangerous,powerful monsters,most of which are near impossible to scout. You shouldn't venture here without powerful monsters. It also has no scoutpost,just a little post you can use to heal your team. The only way of going inside is going into a citizens house.
Good place to have a level grind when the Danger level is indicated unknown.
A good way of training is taking off to infern isle from Domus then walk up to the healing post and just just the whisle skill too keep luring monsters most groups will come 3 at a time with is good for about 300 exp battle that dont even have to take long given u take 1 stronger monster together in with the monsters u want to level up. A good way too get monsters up for syn.
On the isle itself you will find loads of lethal armors which you will fight against when near the healing post.
Try standing in front of it to have a quick full heal in between battles this allows you to unleash your monsters full potentional in every single battle there.