[title
ragon Quest IX Character Classes/Spoilers?] The only really "easy" strategy for the bonus content bosses (and some tough rear end enemies) is this:
CHARACTER ONE is a Paladin, with the "nioudachi" skill. I'm not sure how to translate this, like, imposing stance, but it's the 82 point Paladin skill. It allows this one character to take every attack for the entire party. It should be noted that attacks that hit the whole group will hit this character 4 times.
You want this character to have all the class skill abilities that give it defense and HP.
You also want this character to have either the Metal King Armor set (Metal Slime gear in A-level treasure dungeon chests and then alchemy to Metal King) or the 4-5 star really rare rear end armor set from killing treasure map bosses (Oroboros's shield, Mythical Armor) depending on the boss. Metal King armor/shield blocks 30% (together) of all types of damage. The other set does I think 35-40% but no Earth or Light resistance. If you don't have this, the Gigant Armor you can buy in St. Stein (the castle town with your inn) is passable. It's +15% against fire ice lightning, I think. The genma (atomic?) shield (atomic shield? you take a magic shield and alchemy it with a crystal and then alchemy that with an evil looking crystal) is also 10% resistance against most stuff.
You want 100 points in Shield, and as high dodge as you can get. This character will never attack, so you should get them either barehanded skills or the fighting staff skills so they get +3% dodge.
This should not be the main character, because the "Cheer" skill is MC only and extremely useful.
CHARACTER TWO is a Priest, with all of the +Wisdom class skills from Paladin, Sage, whoever. The Wisdom skill increases the amount each of your spells can heal. You want gear that increases Wisdom. You also want all the +MP skills and the -25% MP Sage 100 point skill. Having 100 in Staff with this guy isn't a bad idea either.
The beauty of this is that this character can use the 6 MP healing spell (after you have the Sage -25% ability) to heal Character One for like 500 HP. If you don't use this strategy, you will be spending 18+ MP a round healing everyone with MultiHeal. You also can cast the spell that raises one character's defense 50% instead of everyone by 25%, on bosses that hit really hard.
This also should not be the main character.
CHARACTERS THREE AND FOUR one of which should be the main character are the damage dealers. You want the main character to use "Cheer" every round to build tension for the attacker. The MC should be fast.
The other character uses the Buildup/"tameru" skill (12 points in Monk/Fighter class) and, when they get high enough tension, attacks the opponent. You can do this with a spell if you want, or just attack. Different attacks have different damage caps. The regular attack goes to 9999 I think, but attacks like falcon sword/hayabusa kiri skill and tiger claw skill cap at 1999 per hit. So the most you can do with one attack is only 3998, but a regular attack would do more. I don't know what the caps for spells are.
Also, every boss has a weakness that causes more damage. Use an ability with this weakness for each boss.
The 100 point Battlemaster skill scroll is also really useful, as sometimes you will randomly do 2x the damage when you have it in your character's items.
Some bosses use Ice Wave to remove your tension, so you might have Characters Three and Four just both attacking every round. None of the regular story bosses use Ice Wave.
There are other strategies that work, but given how every treasure map boss is more difficult than the main story last boss, this is the strategy that made it the easiest. Anything other than using a Paladin to defend means you spend tons more MP healing and buffing and all that crap, and many classes (like Mage/mahou tsukai) just don't get much defense and the armor they get provides no resistances.