In order to trigger this sub scenario you need only reach the point in the story where you are forced to enter the Starkfort to rescue Egil. This means infiltrating the Mythril Mines and rescuing the kids held in slavery within. Do that and it will be open to you - though note that it will not appear anywhere on your map when you enter Starkfort. You'll have to find it on your own. (Don't worry, you'll be prompted with plenty of time to spare.)
- Enter Starkfort and walk northward until you hit a cut scene. During this cut scene, Ringabel will pull out his diary and have a look. He notes that there is 'poison' to the west of where you are, which is, needless to say, not very encouraging.
- Nevertheless, heed his words and go up the northwestern staircase form the Starkfort's bottom floor. On the second floor you'll find a chest containing 1000 pg, a robed save dude, and some stairs. Save up and ascend the stairs. Make sure your chosen healer - hopefully a dedicated White Mage - has full MP, because he or she will need it for this next battle.
The title 'Salve-Maker' is a little misleading, as it implies Qada is a weakling scientist. Don't be fooled, as this roly-poly malcontent is plenty dangerous. His physical attack is pretty weak, true, but he'll spend the majority of the battle dishing out some serious attacks. His standard blow is Poison Hi-Potion, which will deal 1,000 points of damage to a single character per dose as well as inflict poison status. After a while Qada will also use Dark Breath, which inflicts as much damage as Qada has lost in the battle on one of your party members - which is to say that, in the later stages, it can reach 9,999 damage. Ooooouch. This will be an instant kill against one member, and you can't really avoid it.
Also? Regeneration. Yep, Qada will restore 1,200 HP per turn thanks to Water of Life. You can Dispel this, of course, but he'll put it back on. This means having to deal a minimum of 1,200 HP each turn to actually get anywhere in the battle unless you have a White Mage (or a very beefy Red Mage).
Put one of your characters on permanent White Mage healing duty, as per usual in boss battles, and have the rest of your characters wail constantly on Qada. Ideally you want to at least double the amount of HP restored each turn, if not more, if you want to stand any chance of beating him. Specials are really good at this if you've developed Norende by an appreciable amount; lacking those, just beat on Qada with your best attacks, as he has no elemental weaknesses. A lengthy battle, perhaps, but not impossible by any stretch - though only if you get rid of Water of Life each time he uses it. Seriously, it makes the battle sooooo much easier.
- Beating Qada will earn you the Salve-Maker asterisk, which allows you to use Qada's attacks. It's quite a handy support category if you spend cash on items often. You'll also have a Party Chat to watch in the aftermath of the battle.
(Quick note: This battle's aftermath changes a little depending on the order in which you complete sub scenarios. If you take this one on first, the encounter with Kamiizumi later will change; if you take it on second, this will be a little different. I kinda recommend taking this scenario on after going for Kamiizumi, as you'll be substantially stronger by that point, but it's up to you.)