@namhoang909 - Until you roll Sue or Elizabeth from gacha, Fran is your go-to vampire hunter. Her skill increases her HP, ATK, and DEF. Ada's skill is the same as hers so I would feed her to Fran if you already have good rogues.

@Mtkut - 4* challenges are more difficult than normal since they only give you one HP and the enemies are much harder than usual. Adding to that, there are certain gimmicks that come with each map like only male/female units allowed, no physical/ranged units allowed, a certain type of unit gets their UP cost tripled, etc. The only reward you will get is SCs and no unit drops whatsoever. Until you get a good core team (at least maxed CC'ed), I would suggest avoiding these. And considering how an item collection event is coming up, I recommend saving your SCs for that, unless samurais aren't your fancy. But then again, why skip out on a free black unit.