Ivan and Isaac get basically opposite class selection abilities. To have both of them in would basically represent the entire GS class options. Which... would be cool... but damn it's hard mapping out a GS character's options.
Here's my attempt. Tables could make it more space-efficient.
(V=Venus, Ma=Mars, Me=Mercury, J=Jupiter)
Floor 1:
Available Djinni: None
Wind Seer series skills:
Whirlwind
Ray
Ray is good GT damage for this floor.
Floor 2:
Available Djinni: 2V, 1Ma, 1J
Flint - Venus/physical damage
Forge - Mars - Attack buff +67, MT (25%)
Gust - Jupiter/magic damage
Granite - Venus - Halves damage taken, MT
Wind Seer series skills: 0-1J
Plasma
Storm Ray
Sleep
Plasma is great full MT Thunder damage for the floor. 90% GT Sleep
Pilgrim series skills: 1Ma
Slash
Wind Slash
Plasma
Plasma is great damage for the floor, Slash series is wind-elemental GT physical damage should Ivan need it. Pilgrim class gets decent ATK boosts, so this damage is passable, but Ivan's better off with spells.
Seer series (Seer, Diviner) skills: 1-2V
Cure
Bolt
Growth
Flash Bolt
Cure Well
Mad Growth
ST full healing (150HP is full at this floor)! Above average magic damage (Thunder and Grass/Earth)
Floor 3:
Available Djinni: 2V, 2Ma, 3J, 2Me
Breeze - Jupiter - Resistances +40, MT
Fever - Mars/physical damage, inflicts Delusion ~100%
Fizz - Mercury - Full healing
Sleet - Mercury/magic, drops Attack by 25%
Zephyr - Jupiter - Agility +203, MT (100%)
Wind Seer series (Wind Seer, Magician) skills: 0-3J
Impact
Tornado
Bind
Damage is above average here, ST ATK buff, Bind is 80% ST Silence
Pilgrim series (Pilgrim, Wanderer) skills: 1-2Ma
Bind
80% ST Silence, Damage is above average here.
Seer series (Seer, Diviner) skills: 1-2V
Bind
80% ST Silence, Damage is average-ish here, still has full healing to average, almost full for high-HP types.
Hermit series (Hermit, Elder) skills: 1-2Me
Impact
Prism
Plasma
Bind
Damage is above average (Thunder and Ice magic), plus the always useful 80% ST Silence (Bind)
Floor 4:
Available Djinni: 5V, 4Ma, 4J, 4Me
Quartz - Venus - Fully Revives a downed target to 50% MHP
Mist - Mercury - Sleep ~95%
Corona - Mars - Defense +54, MT (25%)
Spritz - Mercury - Heals 126 HP, MT (30%)
Smog - Jupiter/physical damage, inflicts Delusion ~100%
Vine - Venus - Drops Agility by 50%
Scorch - Mars/physical damage, inflicts Stun ~95%
Sap - Venus/physical damage, heals user for ~100 HP
Wind Seer (Wind Seer, Magician, Mage) skills: 0-4J
Ward
High Impact
Resist
Damage is averageish now, GT ATK/DEF buffs are okay.
Pilgrim (Pilgrim, Wanderer, Ascetic) skills: 1-4Ma
Volcano
Eruption
Ward
Resist
Damage is above average thanks to Eruption, which is MT fire magic. At this point, Ivan has better class options, but if he really needs Fire elemental damage for this floor, this class is available.
Seer (Seer, Diviner, Shaman) skills: 1-5V
Revive
Ward
Blue Bolt
Resist
ST Full Revival! Damage is decent again! Some meh GT DEF buffs. Healing is still decent, but not full here.
Hermit (Hermit, Elder, Scholar) skills: 1-4Me
Wish
High Impact
Hail Prism
Wish Well
MT Full Healing off of Ivan's speed is awesome. Hail Prism is great MT (Ice) damage for this floor.
Medium skills: 3V, 3Me
Bolt
Froth
Cure
Flash Bolt
Cure Well
Froth Sphere
Haunt
Revive
Curse
Blue Bolt
ST Full Revival! Almost Full ST Healing! Crappy Status (though rare!) Blue Bolt is decent damage at this point.
Ranger skills: 3Ma, 3Me
Slash
Douse
Ward
Volcano
Wind Slash
Drench
Bind
Eruption
Resist
Average-ish physical wind damage, Good GT Fire magic damage, some below-average Water GT magic damage, 80% ST Silence!
White Mage skills: 4Me, 3V Ivan's awesome class!
Cure Poison
Prism
Plasma
Ward
Dull
Wish
Restore
Revive
Hail Prism
Wish Well
Full ST Revive! Full MT Healing! Full ST Status Healing! Dull lowers enemy ATK 25% (stackable!), Hail Prism is great MT Ice magic damage for this floor!
Floor 5:
Available Djinni: 6V, 5Ma, 5J, 5Me
Kite - Jupiter - Act twice next action
Ember - Mars - Heals 9 PP, MT (7%)
Ground - Venus - Paralyzes ~90%
Hail - Mercury/magic damage, drops Defense by 25%
Wind Seer (Wind Seer, Magician, Mage) skills: 0-5J
Shine Plasma
Great MT damage again!
Pilgrim (Pilgrim, Wanderer, Ascetic) skills: 1-5Ma
Sonic Slash
Shine Plasma
Great MT damage for the floor! GT wind phyiscal damage if needed.
Seer (Seer, Diviner, Shaman, Druid) skills: 1-6V, 1J
Potent Cure
Wild Growth
Drain
Full ST healing again! Damage is still decent (Grass/Earth or Thunder elemental magic)
Hermit (Hermit, Elder, Scholar, Savant) skills: 1-5Me
Shine Plasma
Break
Drain
Great MT (Thunder) damage for the floor, still has full MT Healing, Break is MT 100% Dispel
Medium skills: 3V, 3Me
Potent Cure
Condemn
Drain
Average damage, Full ST Healing/Revival, 30% ID
Ranger skills: 3Ma, 3Me
Sonic Slash
Deluge
Above average MT damage (wind physical and water magical)
White Mage skills: 4Me, 3V Ivan's awesome class!
Shine Plasma
Resist
All of the previous White Mage skills + great MT thunder damage and an okay MT DEF buff
Enchanter (Venus) skills: 6V (Really more of an Isaac class, but Ivan can load himself up with Venus Djinni to do this)
Delude
Impact
Gaia
Ward
Weaken
Astral Blast
Sleep
Haunt
Curse
High Impact
Resist
Mother Gaia
Enfeeble
Drain
Delude lowers accuracy for basic physicals, Weaken lowers DEF by 40 (stackable!), 90% GT Sleep!, decent MT ATK/DEF buffs, Haunt/Curse are worthless status, Mother Gaia is above-average MT earth magic damage, Astral Blast is decent Thunder physical damage
Floor 6:
Available Djinni: 7V, 7Ma, 6J, 6Me
Squall - Jupiter - Paralysis ~90%
Flash - Mars - Reduces damage taken by 90%
Bane - Venus/physical damage, inflicts Deadly Poison ~80%
Dew - Mercury - Fully Revives a downed target to 100% MHP
Torch - Mars/physical damage, ITD
Wind Seer (Wind Seer, Magician, Mage, Magister) skills: 0-6J
Destruct Ray
Cost-efficient GT damage!
Pilgrim (Pilgrim, Wanderer, Ascetic, Fire Monk) skills: 1-6Ma, 1J
Drain
Damage is above average still, Drain is ~10% damage/restore. Useless.
Seer (Seer, Diviner, Shaman, Druid) skills: 1-6V, 1J
PsyDrain
Damage is average-ish here, Full ST revival/healing still, PsyDrain is ~10% MP drainage
Hermit (Hermit, Elder, Scholar, Savant, Sage) skills: 1-6Me, 1J
No new skills.
Damage and Healing are still above average and MT.
Medium skills: 3V, 3Me
PsyDrain
Below average damage, Full ST Healing/Revival, 30% ID
Ranger skills: 3Ma, 3Me
Break
Average-ish MT damage (wind physical or water magic), MT 100% Dispel
White Mage skills: 4Me, 3V Ivan's awesome class!
Blunt
Blunt is a GT 12.5% ATK debuff, Damage is above average here, MT healing is almost full. ST Full Revival and Status healing.
Enchanter (Venus) skills: 6V
PsyDrain
Basically the same as the previous floor, only damage is about average-ish. And he has the godly PsyDrain >.>;
Enchanter (Mars) skills: 6Ma (Really more of a Garet class, but Ivan can load himself up with Mars Djinni to do this)
Delude
Guard
Impact
Ward
Volcano
Weaken
Astral Blast
Sleep
Protect
High Impact
Resist
Eruption
Enfeeble
Drain
Guard is a 25% DEF buff (stackable!), Delude reduces accuracy of basic physicals, some MT DEF/ATK buffs, an okay DEF debuff, 90% GT Sleep!, Average-ish MT Fire magic damage, below average ST Thunder physical damage
Floor 7:
Available Djinni: 7V, 7Ma, 7J, 7Me
Tonic - Mercury - Heals status effects, MT
Luff - Jupiter - Silence ~90%
Wind Seer (Wind Seer, Magician, Mage, Magister) skills: 0-6J (can add any 7th Djinn)
Tempest
Damage is only slight above average here, that hurts.
Pilgrim (Pilgrim, Wanderer, Ascetic, Fire Monk) skills: 1-6Ma, 1J
PsyDrain
Damage is average-ish now, PsyDrain is slightly more useful ~10% MP drainage
Seer (Seer, Diviner, Shaman, Druid) skills: 1-6V, 1J
No new skills
Damage is lagging, Still a fast, fairly durable healer/reviver
Hermit (Hermit, Elder, Scholar, Savant, Sage) skills: 1-6Me, 1J
PsyDrain
Pure Wish
Full MT Healing again! Damage is average-ish here.
Medium skills: 3V, 3Me (can add any non-Mercury 7th Djinn)
Froth Spiral
Slightly above average MT damage, Full ST Healing/Revival
Ranger skills: 3Ma, 3Me (can add any 7th Djinn)
PsyDrain
Below average MT damage (Water magical or Wind physical)
White Mage skills: 4Me, 3V Ivan's awesome class!
No new skills
MT Damage is average, MT healing is about 70%.
Enchanter (Venus) skills: 6V (can add any non-Jupiter 7th Djinn)
No new skills
Same as previous floor, only damage is below average
Enchanter (Mars) skills: 6Ma (can add any non-Jupiter 7th Djinn)
PsyDrain
Same as previous floor, only damage is less potent.
Floor 8:
All Djinni Available: 7V, 7Ma, 7J, 7Me
Wind Seer (Wind Seer, Magician, Mage, Magister) skills: 0-6J (can add any 7th Djinn)
Spark Plasma
Great MT damage again!
Pilgrim (Pilgrim, Wanderer, Ascetic, Fire Monk) skills: 1-6Ma, 1J
Pyroclasm
Spark Plasma
Great MT damage again! In Thunder and Fire flavors!
Seer (Seer, Diviner, Shaman, Druid) skills: 1-6V, 1J
No new skills.
Damage is below average. Healing is about 70%. Get a new class.
Hermit (Hermit, Elder, Scholar, Savant, Sage) skills: 1-6Me, 1J
Spark Plasma
Freeze Prism
Full MT Healing and 2nd-gamebest magic damage (MT, Ice or Thunder)
Medium skills: 3V, 3Me (can add any non-Mercury 7th Djinn)
No new skills.
Average damage (MT, Ice), Full ST Healing/Revival, 30% ID.
Ranger skills: 3Ma, 3Me (can add any 7th Djinn)
Pyroclasm
Good MT Fire magic damage GET! Pilgrim series actually outclasses this one by having two high-level elemental magic spells. And Enchanter (Mars) has better buffs and status. Only use this one if you REALLY need a dispel ability and Fire elemental damage.
White Mage skills: 4Me, 3V Ivan's awesome class!
Spark Plasma
Pure Wish
Freeze Prism
Awesome MT magic damage (Thunder or Ice), MT Full Healing, ST Full Revival and Status Healing, some ATK debuffs and MT DEF buffs
Enchanter (Venus) skills: 6V (can add any non-Jupiter 7th Djinn)
Grand Gaia
Same as the previous floor, but Gamebest damage here. (MT, earth)
Enchanter (Mars) skills: 6Ma (can add any non-Jupiter 7th Djinn)
Pyroclasm
Same as previous floor, but good MT Fire magic damage. Useless unless you really like Fire Djinni.
Suggested builds:
Healing
Wind Seer (Floor 1)
Seer (Floor 2)
Diviner (Seer2) (Floor 3)
White Mage (Floors 4/5)
Medium (Better speed/damage on floor 6, though not MT Healing)
Hermit OR Medium (Hermit gets Full MT Healing sooner than White Mage though lacks revival, Medium gets Full ST Healing and ST Revival)
White Mage (Floor 8)
Status Whoring/Debuffing
Wind Seer (Floors 1/2)
Magician (Wind Seer2) (Floor 3)
Ranger (Djinni selection allows for the most number of MT Statuses on Floor 4)
White Mage (Djinni selection gives most statuses, Floors 5/6)
Ranger (Djinni selection gives most statuses, Floors 7/8)
Damage
Wind Seer (Floors 1/2)
Magician OR Elder (Hermit2) (Floor 3)
Scholar (Hermit3) OR White Mage (Floors 4/5/6)
Magister OR Medium (Floor 7)
Enchanter Venus (most damage) OR White Mage (2 elements) (Floor 8)
Mix and Match for what you need most for a particular floor!
I refuse to do this for Isaac, too. This took too long. All information compiled from Golden Sun wikia's comprehensive Psynergy/Class/Levels/Djinni lists. No clue about equips here, but they largely don't matter compared to class/Djinn unleashes.