The Bosses in the Closed Lands are much harder than what you've encountered up to this point. Each one of them has attacks that will either kill one party member, or multiple party members, on a somewhat regular basis.
There are a couple things you can do to help prepare for this - but ultimately, unless you do a bit of grinding before each Boss battle, you'll probably lose (and even if you are able to win, it will depend much more on luck than strategy.)
In the online shop, you can convert all your enemy encounters into "SOP points." (5 victories = 1 SOP)
At this point in the game you should be able to generate at least 100 SOP this way. If so, I highly recommend you buy 3 Amulets of Mystery and one Orlov, each of which will cost 20 SOP.
The Amulet of Mystery protects against all Abnormal States and Status Drops. (I recommend 3 Amulets, because you should already have one Unicorn Amulet, which protects against all the same things except for Status Drops. If you have the SOP, you could just as well buy 4, but you only need 3 of them.)
The Orlov will earn you an extra 50% EXP. (Very useful for the bit of necessary grinding in the Closed Lands!)
Before the final Boss battle, you should complete all available side quests, and return to the Elves' Village and trade in your Fossil Plates for one Megin Gjord, plus whatever else looks good. (You'll want to save your game before you trade, because you have to 'buy' the item in order to examine it and find out what it does... and if you Save right beforehand, then if you don't want that item you can just reload and trade for something else.)
The Megin Gjord makes the Deadliness Gauge rise faster, which will be a huge help in the upcoming Boss battles.
You can try your hand at Group and Individual Training if you want to; if successful, the Individual Training rewards include some pretty cool bonus weapons that will be helpful - but you don't have to do that in order to beat Jenowin.
You can - and should - save the Crystal Locations stuff for later, as you won't be nearly strong enough yet.
Skills and Equipment
You should also check against the Skill List, and individual Weapons and Armor lists, and verify that the party members you plan on using all have learned all their skills, and have equipped the strongest weapons and armor available (a few pieces of gear are in treasure chests in the Closed Lands).
If you want, you can also reinforce one weapon (but only one, as it's ridiculously expensive at this stage!) If you do decide to reinforce a weapon - I highly recommend that you wait until you have Denoas' Ash Knuckle Duster, because Denoas will probably be doing most of the attacking in the final battle, and the Ash Knuckle Duster will be his best weapon.
Also, check your available Accessories and be sure to adjust your Merit Points. (I lean heavily toward max HP boosts, and boosts to Attack Strength or Critical Hits, but you may find another approach.)
Finally - make sure you have plenty of health potions and resurrection potions (like 25 or so of each). You may not need them, but it's better to have too many, than not enough. (Especially since you have to go all the way through the Closed Lands to reach Jenowin - if you get there and then find out you need more potions, that means going back to town, stocking up, and then going back - again - through the entire Closed Lands.)
Also, it would be wise to take a moment and watch the opening sequence of LemmyTheLenny's Boss videos - each one shows the party level he reached before facing that Boss. While you don't have to grind to that same level, the closer you are, the easier the battle will be.