Brave Frontier

I suggest waiting for another unit to it on since nothing you mentioned really needs dual sphere. You're gonna want your frog for high BB cost units and/or units who need to have BB every turn (Tridon, Mifune, 7* Starters).

Matah doesn't need double sphere at all. Kuda and Orna are top tier crit buffers, but they don't need double sphere either. If you want to use one on them It depends on who you use more. I prefer Orna since she has a stronger crit buff and the elemental buffs. Kuda is still a solid choice though, especially if he's your only attack buffer or you use him alot in Arena. The only unit from the Dragon batch that needs dual sphere is Sierra, but you already have Darvanshel. She should be able to keep SBB in raid anyways.

Thought about my Tridon but I promised myself not to sphere/burst frog him because he made things a bit too easy. I mean BB1 was good enough for dungeons I had trouble with when I ran it the first time (I.e. the lv 3 ggc dungeons. I actually brought him to Eze and surprisingly he survived thanks to that stupid overpowered shield). I have a Michele but I didn't level her because I had Orna and her dual element buff as well as having a different offensive buff. It just gives me reassurance that I don't really need to use the frog right away since the units that need a sphere the most are already sphere'd. :b