Getting Frenzied Slash or Headbutt depends wether you prefer a more AoE dps or single-target dps playstyle.
The biggest problem with Headbutt is the total lack of AoE atk ratio, which makes it horrible against a big quantity of monsters. On the other hand, Frenzied Slash hits a fixed amount of targets, which is much better than +AoE atk ratio. Hitting 19 targets means about 38 AoE atk ratio (medium-size mobs have 2 AoE defense), which is super good and doesn’t rely on Centaurus Cards.
Of course Headbutt deals very high damage to bosses and gives tons of stuns too. If you’re more into bossing it’s your best Murmi skill for sure.
Between Frenzied Slash and Scutum, I think Frenzied Slash is even better at the moment. It has a bigger area, hits much more targets, stuns targets and deals more damage, all that with the drawback of 5 extra seconds cooldown.
Frenzied Slash used to suck before Re:Build and Scutum was the bread and butter for Murmillos. Now on Re:Build however tables have changed.
Evade Thrust is horrible now. Too little skill factor and AoE/AoE atk ratio.
NOTE: Btw it’s not written anywhere, but Frenzied Slash gets +50% dmg from Cassis Crista as well.