Since there doesn't seem to be a walkthrough for the last quest (and I've spend some time in that damn castle) I thought about making this one for you
I assume you have finished every quest possible. If not, do it!
Finish any business you have in Metrolia, stock on every possible medicine (having a number of Ocean Love, Royal Pirahnas and full stock of Pink Apricots is highly recommend), look one last time on Metrolia and off we go. While on the road to the village, Random mentions he wants to do the last quest in his old squad, meaning only Sarah, Luciana and Coryool (he'll be VERY useful later on!). You don't have to walk around the village as there is nothing really important, just visit the town mayor (big house on the right) who will give you the permission to enter the forest. There are a few alchemy ingredients scattered around the forest but you won't have the chance to use anyway so just go north, north, right (unless you want to fight a giant spider. It gives a teszrandus clutch but again, you can't even use it) and north again to the cave where the "monster" (a common lash wolf) is.
You don't actually even have to attack him as Luci can't resist being the hero for once and finishes it before it has a chance to attack. Quest complete! While on you way back, your patch gets blocked by a magical crystal - just visit the empty house and get rid of the demons inside. End of the game?
Nope, now the real fun has started. While you are on your way back to Metrolia, the town gets slaughtered by an unknown wizard. Luci feels the demon energy and loses all of her saved beacons (not that it's much of an issue now). Go to your HQ, rest and double-check your equipment. Head to Metrolia woods, now infested with a demons (fortunately, pretty weak). The most annoying of them is Royal succubus who not removes equipment of your party but uses a suction spell as well. Fortunately she has very low HP and dies after one good attack. Also watch out for Hell Murderers who can curse you. Again, no real point in exploring the woods. Keep walking north until you'll find a save point and a split. Go the right path and north again. See the huge numbers of Gloors? Head in their direction (fortunately they are very weak) and then go north. You should be next to the castle.
Heal up, check you equipment and use the right doors (there is nothing behind the left doors beside a large groups of enemies). Having a protection from suction is a must here, both Catharsis demon and worms of hell use a good deal of suctions spell. Another bad news? Worms of hell attack in groups. But it seems they are vulnerable to death spells. Rest of enemies shouldn't be a problem at this stage. From the right entrance, go north,right and north again until you see a stairway, go up. Go down, there is a save point on a next screen on the right. Now continue down and turn left. Be careful now - there is a gap in the floor which drops you back to the castle entrance. Go north. Before jumping to the hole, heal up and check your equipment - the real fight begins.
After a short cutscene, the unnamed Demon tries to kill you with a death spell but to no effect. Coryool's aura is blocking the spell! (If you didn't take him, you would be dead meat as there is no other way to avoid the death spell!). Since you managed to annoy the demon, now it's time for a round of battles!
Fight 1 - the wizard.
Pretty weak fellow, after you cast two times Grand Protection his attacks deal no damages. Just use regular attacks.
Fight 2 - electric drake.
Still rather weak but now regenerates 800hp per round. Double strike weapons come in handy here, if you got the "Hungry pixies" spell, use it too.
Fight 3 (he's annoying like Dagayel, honestly) - Huge Thing, regenerates 1200 HP per round.
Same strategy as previously.
Luci realizes the demon is a morph demon kind and there is a low chance of defeating him, since he can change his form immediately. Fortunately, the wounded man informs you there are legendary relics in the chest behind the demon and helps you get them. They are pretty easy to find in your inventory - just look which item has question marks all over them
One detail though: think whether you want to change Ulm-Warrior into Inverness sword as it strikes only once.
Fight 4 (the last one, thankfully) - Chaos Eyes.
Prepare for quite a long fight (and couple of replays). He has three main attacks : Great Havoc (very powerful, can easily knock your party down. Ocean loves will come in handy here!), Call (summoning a weaker demon) and a dark spell that doesn't do much damage. Suction spells don't work on him, neither does Coryool attacks. I used Random and Sarah for double hits, Luciana for Grand Protection/Shield and later Lord of the Down/Pyramid spell and Coryool for slash up both on Sarah and Random and healing items (he's the least affected by all of Chaos eyes attacks so even if Luci, Sarah and Random get blasted by Great Havoc, Coryool can use Ocean love and bring them back). After many rounds, he should fall down. Yep, that's the end! Now you only have to watch a rather downing ending where after fainting Random has a dream where Metrolia and all it's citizens is brought back to life and act like nothing happened.
Game complete! Now you can go straight to Laxius Force!