Easy:
Level 4-Falcon
Level 7-Anoat
Level 11-Yuzzem
Level 15-Brigia
End Game-Greedo
Normal:
Level 4-Biggs
Level 7-Kaiburr
Level 11-Mynock
Level 15-Degobah
End Game-Mimban
Hard:
Level 4-Ackbar
Level 7-Fornax
Level 11-Bespin
Level 15-Kessel
End Game-Ogana