Simply sell the unit as you would always do it. Gold rarity units and above will give you additional rainbow crystals as mentioned above. Keep in mind that event/TP units drastically reduce the amount of rainbow crystals you get. I don't know the exact amount, but I do know that golds and plats net you a value below 10 and event blacks give you 20.
Aigis Teams (10/23/2016)
1st: Nutaku Aigis Main Team - 2nd: Alt. Nutaku Aigis Main Team - 3rd: DMM Aigis Main Team
Aigis Unit Collection (06/02/2016): DMM | Nutaku
Aigis Story Guides:
Eastern Wind: http://harem-battle.club/millennium-...ion-guide.html
Magic City: http://harem-battle.club/millennium-...ion-guide.html
Jungle: TBA whenever Nutaku implements it
Makai (Hell): TBA whenever Nutaku implements it
Other browser games I play (or played) outside of Aigis:
Druids do not need to be CC'ed. They are special classes, so you could just max her level and affection and awaken her.