We need an SP item

Just get 3 tanky units (or 2 of them and dancer, or even one if you have nice map with bottleneck), without sloted weapons, and go on one of 35+ maps, then make those 3 tanks do their jobs, and heal them like crazy monkey. Also make your healer kill all enemies but one, then heal till you stop getting sp (i think there is a limit, just like exp) and then finish the map)

And i agree, sp item would be nice, but not in a form of direct fill up. Just like that new water item works, seal or what is it, it gives doubles. My Fjorm was getting 24 sp per kill (x2 from double sp, x2 from water boost x2 from S!Robin's valor).

I agree, sometimes it's a pain to try to get the SP you need, especially if your unit has to learn both a new weapon tree and all new skills. It took me forever to finally finish my Linde, and I'm not anywhere near finishing my masked Marth.
They should have some sort of special reward, one that doesn't come out that often, so players can't abuse it but at the same time it remains useful.

Feel your pain. Went through it with my Y Tiki, when I literally replaced every one of her skills, including her breath.

I settled for using TT to do the SP farming. That's basically easier and get some other rewards.