Finally I had some time to look it up.
Indeed, it's just a case of a unsigned integer overflow, the game doesn't not check if the subtraction (AttackerLevel * 20 - [ TargetDef / 2 ]) goes negative. Following Forte's lead it looked like it was deliberate, oh well. About the combo thing:
When used in a combo, the amount of hits the breath attack does (8011E0B9) depends on the "combo level" (8011DF50)
IF COMBO_LEVEL == 0, HITS == 6
IF COMBO_LEVEL == 1, HITS == 9
IF COMBO_LEVEL == 2, HITS == 12
The formula itself is a loop (0 ... HITS) whose iterator is stored at 8011E0B8, and, as you know, the damage of each hit is increasingly weaker. The game achieves that by the reducing the first term of the subtraction, therefore incidentally increasing the chances of our overflow. Being the MODIFIER stored at 801EAA90 and later at 801C6E58 with the value of 6, the first term is:
IF ITERADOR < MODIFIER+3
IF ITERADOR < MODIFIER
(AttackerLevel * 20)
ELSE
(AttackerLevel * 10)
ELSE
(AttackerLevel * 5)
The MODIFIER is 6 in case of the DragonBreath Attack, but each breath attack seems to have its own value and but I didn't look further than that. The formula could be quite different anyway.
I've spent a lot of time the last few weeks trying to break this game somehow and I finally did it. I know that it was noticed a very long time ago that the damage done from Aura DragonBreath attack can be preposterously high in certain conditions. It was not before Forte in his damage formula thread that it was exposed that a interesting condition was checked:
If TargetDef / 2 is greater than AttackerLevel * 20:
BaseDamage = 65536 + (AttackerLevel * 20 - [ TargetDef / 2 ])
Basically, if Ryu level times 20 is less than half the enemy defense, the damage is big, very big. Well, it turns out that this is actually very hard to satisfy. I am not sure why it was never exploited (as far as I know), but you have to be really determined to deprive Ryu of any exp, specially when fighting Ight, when Ryu transforms into Kaiser, remember? You have to kill Ryu (twice!) to prevent him gaining exp and thus transform what would be a free fight into a very tricky one. Why make such a effort?
Anyway, this is a video of me one shooting the toughest enemy in the game, Rider (defense: 210), with a level 6 Ryu:
Link to video
Keep in mind I am not using any GS code and played the game start to finish to verify and test this.
Ruy gains xp from Fou-lu battles so I made a excel sheet to track the party levels from each boss fight. I then concluded that... you level simply never gets low enough compare to the bosses stats (by far...). We need a few tricks:
First, we can raise the enemy defense ourselves with Cray's Protect ability or the King Sword. However, since this buff is limited to +100%, the condition is still too demanding making it useful only for the latter three bosses and enemies with enormous defense, like the Rider.
Second, you probably realized that if the enemy is defeated with the Breath attack, Ryu will be alive and gain xp making this only usefull once. Or it would be if not for the Monopolize skill (caster appropriates all xp) which is learned from the master Marlok.
Third, when the breath is used within a combo, somehow, and it bothers me a lot I don't know why, the condition is evaluated differently, making it easier too fulfill it so that we are able to kill EVERY boss in the game if just a single hit (we still have to buff its defense one or two times most of the time)
A few notes:
- If you want to try it out, Ryu level is in address 0x8011A956. Must be set before the battle begins.
- The dice I and II are a exception, their move Stasis blocks all combos for 3 turns. You have kill one of them "normally" I assume.
- I checked all others breath attacks and seems DragonBreath is special in this regard.
- We get access to the DragonBreath attack only after fighting Ight.
- If you don't miss the treasure in Ludia at night, Monopolize is made available right when you need it.
- The bot xp grind is no longer needed (hooray!) and Treans can be killed in the Abandoned Village for a very early Nunchaku weapon for Cray. If a TAS was to be made, I believe this would be useful for the dice I and II. (see this video )
- I don't think a RTA is feasible, but maybe Ershin's Stand Out skill may be useful to prevent your frail Dragon being killed?