Cherche: is Slaying Hammer good or does she need Brave Axe+?

Mmmm. Slaying hammer could be good, but with -spd, may as well go Brave Axe. Her speed's pitiful anyway, even with flier buffs.

If you REALLY like using Cherche, go with a Brave Axe+ and then do the Desperation 3 +Brash Assault 3 Seal. Get her below 50% HP and go insane. I use a Brave Axe+ Cherche without that combo and she's already really good.

If you had 5* Cherche and you don't want to "lose" a 5* unit for her, you can use Brave Axe and give her seals, skills to increase her atk. I think with good teammates, she is still good with Brave Axe.
My Cherche with Brave Axe is good.

As you are F2P I would recomend Slayer Hammer +atk, Defiant atk or Deathblow, Desperation 2, Some Hone or Spur that you want and the BrashAssault Seal, mimicking the famous brave build, with something much more cheap and obtainable from easy to get units that are in the 3-4 pool. Defiant atk is prefered because the synergy w/ the other skills.

Slaying Hammer+ could be a good secondary weapon to swap with Cherche's main Brave Axe build, now the reason being is that sometimes events would want for you to have some sort of armor slaying weapon to make quests easier, like the Christmas banner we're getting is consisting all armor knights and it could be worth considering to give Cherche slaying hammer with her huge attack stat.