So I finally managed to beat the Zant* without using any seals. It took quite a while.
(*btw, is this his name, or just a demon-type, as in "a/the Zant"?)
If I were to suggest something, I'd say you should stock up on healing potions/oneberry juice. I only had two potions, but over 20 bottles of juice, and I used them all up, regretting I didn't take at least 2-3 more.
I took the best equipment I could, but I didn't go out of my way to make hundreds of ducats* to buy all those "uber" items, since I thought it would probably turn out to be a little game-breaking later. (*it's possible)
As for strategy…
I had Forgrimm, my main character, and a summon (bear) focusing on the demon. Most of the attacks barely made a difference, but by using whetstone you always do at least 1 point of damage when you hit. Mighty Blow rarely hit, and when it did, it would only cause a few points of damage, but I kept Forgrimm using it, since it was his only useful skill for this battle (he actually had the Berserk skill, but I totally forgot about it, so I don't know if it would prove useful, lol). As for my character (sword+shield warrior) the most efficient skill was Lunge (I think that's what it's called in the English version) since it (partly?) ignores armor. When I was lucky it would deal around 10 dmg.
Fayris was the only caster in my group, but the mana burn aura effectively prevented me from casting any spells during combat, so I cast what I needed right before the battle. Immediately after it began I managed to cast one Thunderbolt, which dealt a nice amount of damage, and then mana went bye-bye.
I only had one mana potion, so I saved it for the time when my summoned bear expired to summon a new one (if it's killed before it actually times-out it's probably a bad thing). If had more potions/ways to replenish mana, the battle would likely be a lot easier!
Anyway, I had her standing on the far side of the circle, shooting with fire arrows. They are essential; most of the regular hits barely caused any damage, but the fire effect causes constant damage, so it's important to keep the Zant on fire. 200 arrows should be enough, but it won't hurt to have more. The ranged battle skills are also important, Marksman sometimes caused even 20 dmg.
When the demon was on fire, I couldn't use Marksman yet, and felt like I had a spare moment, I would have Fayris set up a bear trap.
Oh, as for mana: there is a certain costly robe that has some mana regeneration, but I don't know if it wouldn't be just offset by the mana drain aura. I didn't try it because I bought a shield for myself, but if it does grant you some mana during this battle, it would be darned useful, especially if you're playing a mage, I guess.
As for Cano, just keep casting his special skills as soon as they regenerate - Stardust, and that offensive spell. While I didn't invest in the traps skill for Cano, it was unlocked for him, so he *could* set them up as well. It's a little risky, since he can take a lot of damage when he fails, but I found that most of the time he would be successful anyway. Now, as he also has some nice combat skills like Lunge and Bleeding, I was often tempted to join combat and backstab the demon, but unfortunately it often resulted in quick death for Cano. It's safer to keep him away and casting the spells, but if you're feeling lucky… well, I managed to hit for 16 dmg once with Bleeding (the bleeding effect doesn't work of course, but the damage counts!).
One more thing, when I *did* use Cano to stab the Zant, the demon would immediately target and pursue him. If you start to run away with Cano soon enough there should be enough space between the two that the demon won't be able to reach him while running. You can then run around the platform while other characters shoot, try to attack with melee weapons, or heal themselves. I think I made over ten rounds and the demon still wouldn't target anyone else. It was rather silly, but unfortunately I slipped up eventually, the demon reached Cano and killed him in one blow. D'oh.
Bear traps are nice, they do around 14-17 dmg, but I found I didn't always have the time to set them up. Only three traps can be set up at one time - naturally, set up three in advance,
before the battle. It's best to put them closely together. When I had 2-3 ready, I would lure the Zant into them with my fighters. Perhaps devoting Fayris and Cano to setting the traps constantly would prove to be more efficient than them attacking the boss, but I didn't go as far as actually testing it. In any case, I had around 50 of them, but used only 20 I think. If you're getting a little desperate, or feeling lucky, you can also set up the traps right under the Zant, for instant gratification - I did that successfully a couple of times with Fayris - somehow she avoided all of the attacks.
The Zant has a nasty area attack which can cause a lot of damage to all surrounding characters, and has a chance of knocking them to the ground. For this reason, remember to keep the other characters like Cano far away to avoid unpleasant surprises. However, this attack often missed, causing no harm. Then again, when I was unlucky he used it three times in a row, hitting each time, and causing me a lot of trouble. The teleport combined with a back attack skill that he has can also be very dangerous, but there's nothing you can do about it.
The battle itself is long enough that any fallen characters actually have a chance of getting up from the ground during the combat. I think it's only possible if the fighting is going on far away enough from the body of a character. When the Zant does the teleport backattack, he disappears for a split second, and at that time a fallen ally may or may not regain consciousness. It can be life-saving if you messed up earlier and a character died.
Anything else? Remember to use whetstones, healing powder, and whatever relevant buffs you may have, including strenght and courage potions for Forgrimm or yourself if you have any. The most important thing is to keep the Zant under constant damage.
Near the very end of the battle I disregarded safety and went all out, attacking him any way I could, and happily emerged victorious.
(However, I must mention that in one attempt, I fought the boss for a veeery long time, and when he had like 1% of his health I still got killed. You can imagine I wasn't very happy with that. So be careful.)
It was a nice boss battle, but it was just loooooooooong.
Well, I hope this will be useful to someone when the English version comes out. Cheers.