I did some research similar to this topic for the Four Destroyers because I want them to actually have more to do in their chapter than is actually available.
So, here are the boss battles, that I know of, were you fight any of the Four Destroyers. The first list is the boss battles that are normal gameplay for the Four Destroyers; the second list is the boss battles that Hugo, Chris, and Geddoe can go on and win (for all of these Hugo, Chris, and Geddoe battles, I can't remember which battles are Game Overs and which battles will continue the storyline):The Four Destroyers Playable Boss Battles
1. Alma Kinan (Chris Ch. 3)
2. Ancient Highway (Hugo Ch. 2)
3. Serenia (Beginning of Hugo, Chris, and Geddoe Ch. 4)
4. Great Hollow (Geddoe Ch. 1)The Four Destroyers Nonplayable Boss Battles
1. Mt. Senai (Geddoe Ch. 2 & 5)
2. Kuput Forest (Chris Ch. 3 & 5)
3. Duck Clan Village (Flame Champion Ch. 4)
4. Cyndar [Sindar] Ruins (Flame Champion Ch. 4)
5. Ceremonial Site (Flame Champion Ch. 5)
NOTE 1: I realize, just today in fact, that this info can be culled from this Suikoden III Gameplay Guide: SIII Bosses
. That said, I think did find out this information on my own when the game first came out, though, and only used that guide as reference check for the chapter. Doesn't really matter anyway because I don't care about getting credit, this note was just in the case of full disclosure.
NOTE 2: Anytime 'Reincarnation' is reference in that guide, it should probably read: 'Rune Incarnation'. That's because, in-game, the abbreviation is 'RIncar' not 'Rincar' (i.e.: the I is capitalized rather than lowercase). Also, 'Rune Incarnation' is probably a better fit because the definition of incarnation is to, "embody in flesh", which better describes what the True Elemental Runes do in the game.