Idly, a quick Etrian Odyssey 2 musing post:
Ronins: Synergize with Troubadours, Hexers, Landskigntrg
Protectors: Synergize with War Magus, Medic, pretty much anyone else
Landskugrd: Synergize with Gunner, Ronin, Alchemist
Gunner: Synergize with Troubadour, Hexer, Landskptjg
Dark Hunter: Synergize with Hexers, Protectors, Survivalists, War Magus
War Magus: Synergize with Hexers, Dark Hunters, kinda Survivalists, Protectors
Troubadours: Synergize with pretty much anyone (especially Ronin/Gunner/Landksugn/Hexers)
Hexers: Synergize with pretty much anyone (especially War Magus/Dark Hunters/Troubadours)
Medic: Synergize with Protectors, Survivalists, pretty much anyone else as well except War Magus.
Alchemists: Synergize with Landskirgwegf, Hexer
Survivalists: Synergize with War Magus sorta, Beasts, Dark Hunters, Protectors sorta
Beasts: Don't synergize except with Survivalists.