Alright, gotta give you people tips regarding fusion tricks:
-If Okita is fused with either Masamune or Kazuhisa, she'll get their action points.
-If Maria is fused with Kasumi, her speed will increase to 9 (unless it's already at 9 or more).
-If any of the four Takeda generals is fused with each other, they'll get Extreme Fuurinkazan, which is an even stronger buff.
-You need to fuse Rance with Shouen if you want to beat Nami for a SAT bonus.
-If you want to improve a character that would lose their more useful normal attack upon class change (like Omachi's lightning, Gandhi's Super Fire Blast or Reset's arrow barrage) you could use the Fake Angel or Kaito scrolls. That way, you'll be able to buff their stats without losing their best technique.
-If you can change a character to a cheaper unit type and back and they have no skills that would get replaced, do it so you can increase their numbers with greater ease.
-Dragon Knights, Iron Knights and any unit with the Uruncel Blade will attack twice even if the target is dead. Exploit this to further empower Beezeleye.
-You can just fuse Silver/Gold Angel/Beat Angel/Super Shinobi units with other units that have no passive to spam their effects.
-Rare abilities that give you a buff to your level cap will keep it even if you replace them. However, I don't know if abilities like Angel Eater will have their secondary effects replaced.
-Likewise, new secondary attacks obtained through rare abilities like Big Banchou will remain even if you change the passive.
-Rance doesn't lose his Archfiend powers if you fuse him and he gains a new ability.
-Since Rance gets a higher percentage of a Fiend's levels during fusion, you should level the Fiends a little through dungeons and so on. I believe the Human Experimentation passive doesn't let you get a 110%.
-Levels gained by fusing while someone has Human Experimentation are rounded down. A lv39 unit will only give 3 levels, so go for that 40.
-Some unusual passives will need you to fuse twice so the target owner can use them. An example would be turning Kabachahn into a warrior/foot soldier/whatever so Omachi can use his Lightning Lord passive.