I'm guessing you started recently and wasn't around last raid event. During those events fusion Kirins and Sea Fairies are plenty since every time you beat a boss you get a prize for discovering, one for partecipating, one for MVP and one for last hit. And you're gonna beat lots... After that event I had over 300 Kirins and 40 Sea Fairies left unused because I already had raised every card to max.
So definitely stop being frugal with your Fairies.
During this event on the other hand is harder to level characters. But I don't think fusing N cards for slots is worth your time. You need in average 4 cards to get a 2-slots (one base and 3 fused) and 16 to get a 3-slots. If you just fused those 16 to your 3-slot you'd have 15% of unlocking the fourth anyway.